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Deepu Dharmarajan -
Posted 49 days Ago

TRAIN DESCRIBERS

CONTENTS 1. Introduction 2. Berth to Berth Train Describers 3. Facilities and Terminology of a Train Describer 4. Train Describer Planning and Design 5. Train Describer Stepping Tables 6. Processing Systems 7. Display Systems 8. Signalling Inputs 9. Peripheral and Ancillary Systems 1.  INTRODUCTION As well as knowing of the existence  of  a train  within  his control  area, the signalman  needs to know something about its identity. The type or class of train  and,  when  necessary,  the route to be taken at the next junction,  is  adequate  information  in  mechanically  signalled areas (where the area of control is very  localised).  Train  movements  are generally  recorded in the Train Register, which also provides a permanent record  as  a  reminder  to  the  signalman and to aid investigation of any incident. The train register, the block bell and a working timetable are usually adequate  for a small  signal box. With power operated signalling and automatic signals it became  possible  for several trains to be in the sections between signal  boxes,  creating  the  possibility  of confusion. More information was required. Magazine type train describers were introduced. They operated by transmitting a series of pulses, similar to an impulse dialling telephone, along just one or two line pairs. The display gave a brief description of the  trains  approaching  on  each  line and  the  last  train  departed on each line. Up to three approaching trains could be displayed and  further  stored  if necessary. The form of the display was an illuminated light against a fixed description label. 2. BERTH TO BERTH TRAIN DESCRIBERS The limit was eventually reached where the description of trains approaching became insufficient. Trains would remain on the signalman's panel for considerable  time.  Several trains could be under the control of one signalman. It was therefore necessary to provide a description of each train as it progressed through the control area. An alphanumeric display associated with each signal on the panel generally provides sufficiently detailed information. The characters may be used to indicate the class, route and sequential number of each train. Must railways haYc a st;md:nd fmm:11 for tuin idrntific1tilln The BR. s·stcm consists of Train describers may be restricted to displaying train  numbers  only  in a valid format or may be capable of displaying any combination of characters. Together with the 4-character berth display system came the automatic stepping of the description along the control panel, controlled by the passage of the train. The basic train describer system therefore consists of the following:-   A facility to store the train description or identity in a standard alphanumeric format. Each store, usually referred  to  as  a  berth  or  address  is  normally  associated  with a particular signal section. Each address may be empty or may hold a   A means of moving train descriptions  from  berth to berth in step with the movement  of   A means of displaying the descriptions held in some or all of the berths. This  may either be on the signalman'panel or on a separate   A typical train describer  display  for a single berth would  appear as below when incorporated  in the signalman's panel. The transfer of a train description from one berth to another is known  as a STEP.  This  is usually initiated by the passage of the train. To ensure that the signalm211 is awa;e of a train which has not had a description allocated, a train without  a description  when  it moves  from one signal to another will  be  allocated  a  not  described  special  description  (N.D.S.D.).  This is often displayed as "****" or"*---".   If the train  description  is stored  but  is not  required  to be displayed,  the berth  used  for  this  is known as a blind bet1h or blind store.   Cum crsc.:ly, some sign:il :,,cct1u1h :ouch 1 SlJ.tiun pLlliurms may oltcn be ;_i_Jluc:HcJ r:hHc.: th:rn one berth if two trains regubrl y use the platform at the same time. 1.             FACILffiES AND TERMINOLOGY OF A TRAIN DESCRIBER   To provide the operators with useful information,  the signal  engineer  must  define precisely the manner in which the data (train descriptions)  are to be handled.  To enable  that definition to be precise and unambiguous, train describer terminology  has been standardised.  Some of  the common terms are defined below.   3.1     Signal Boxes   A signal box large enough to require its own train describer system will be described as the main box.   It will normally send and receive trains and  train  descriptions  to/from  other  signal  boxes with their own train describer systems. Each of these is referred to as an adjacent box.   Due to the limited amount of traffic or small area  of  control,  train  describer  working  may not be required throughout a railway. Block  bells or other similar methods  may  be adequate. A signalbox at which  this  transition  occurs  is known  as a  fringe  box. This  box  will  have a display of trains approaching from the Main Box and be able to send descriptions of trains proceeding towards the Main Box using equipment which is part of the Main Box Train D,escriber system.   Within the area controlled by the Main Box local control facilities may be  retained  for shunting in depots and yards or supervision of level crossings. These are referred to  as shunting boxes or crossing boxes and generally have similar facilities to a Fringe Box. ,,   The transfer of train descriptions from one box to another will be by transmission from the originating box and reception at the second box. This may  occur between  a main  box and any other box with which it communicates.   Types of Berth   Most berths are associated with a signal and will hold and display a single description. The description will step in and out of the berth with the passage  of  the train  past  each signal. Most train describer systems will require a few specialised berths which operate in a slightly different manner. These are described below.   Ripple Berths   Certain signal sections may be designed to permit more than one train in the section. An example is a station platform where  two  trains  are joined.  Consequently,  extra  berths  may be provided up to the number of trains allowed in the section. The berths are  known  as RIPPLE BERTHS and a train description will automatically  occupy  the  foremost  vacant berth. Ripple berth situations may include Blind Berths. Simple Ripple Berth Situation Any description arriving in BOOS is required to move forward to A005 if the latter is clear, otherwise it should move forward when A005 becomes clear.   Where there is a series of automatic signals on a section of plain line,  it  may  not  be  considered necessary to  have  a  train  describer  display  for  every  signal.  The simplest  way of reducing  the number  of displays  is to have a series of ripple  berths with only the first one  or two actually displayed. It is usual for the last automatic  signal  to be excluded  from  the above arrangement to give a better indication of trains approaching the controlled signal.   Series of Ripple Berths for Automatic Section Shuttle Berths   Where a signal section is bidirectional and more than one train could occupy  the section (e.g. station platforms), multiple berths will be provided. These are known as shuttle or p'ush-drag berths. The method of operation varies between the two types of  berth according to the particular local requirement. Although each berth is associated with  a signal, each may be used for movements in either direction according to circumstances.   Typical Shuttle Berths Shuttle berths  behave  in a simibr  manner  to Ripple  berths,  but on  bidirectional  lines.  When a description steps into 0006 from 0001, it  will  immediately  shuttle  forward  if  0003  is  empty.   When a train is present  in  the  platform  with  its description  held  in 0003  berth,  and  a route is set for the train to proceed from  6 signal  it  is  necessary  to shuttle  the  description  back from 0003 to 0006 so as to be in  the  correct  berth  for  the step 0006  to 0002  to occur  when the train proceeds.   Also if a description of one train is held in 0003 when a second train is signalled into  the platform at  that end from 8 signal, the .first  description  must shuttle  back from  0003 to 0006  in order to make room for the second description to step from 0008 to 0003. Push - Drag Berths   The example uses the same layout and arrangement  of  berths  as for  the  previous  example. The  stepping  logic  is slightly  different.                                                                    ·   For trains  running  in the down  direction  from  1 signal  the description  will  step from 0001  to 0006 as normal. If 3 signal is at red the description will stay in 0006.   If however 3 signal  is already  showing  a proceed  aspect  the step into 0006 will immediately be followed by a step from 0006 to 0003.   If 3 signal is cleared after the description has entered  0006,  then  the  description  1s DRAGGED into 0003.   If a second train is signalled into the platform from 1 signal, and the first description has remained in 0006, then setting the second route from 1 will cause the first description to be PUSHED into 0003.   Arrangement of Push-Drag berths Operator's Control Unit   Apart from simply observing the train descriptions provided by the train describer, the signalman must be able to insert, amend and delete train descriptions. He may also be provided with certain additional facilities (e.g. to find the location of a particular train).   On train describer systems incorporated in the signalman's control panel,  the  operator's control unit (OCU) is the means for the signalman to communicate with the train describer. This generally consists of  at  least  three  displays,  an  alphanumeric  keyboard  and  a  number of function  keys.          ·   Typical Operator Control Unit Layout for Control Panel Main Functions   By using the  alphanumeric  key  pad,  the  signalman  is  able  to  SET  UP  information  into  the train  describer.  He  enters  the  signal  address  berth.  which  will  normally   be   the   sigr:al number. He CJn then:- If a train description is also entered immediately after setting up the signal address the signalman may:- OCU Indications and Alarms   If a train moves within the train describer area without an associated  description,  it is known as not described and, as such, causes the signalman  to  receive  a  not  described  alann (NDA). This comprises a brief audible alarm, together  with a flashing  indication which  may be acknowledged by pressing the NDA button.   In addition to the visual and  audible  alarms,  the berth  address  for  the Not  Described  train is displayed in the OCU Alarm Address berth.  This  information  will also  be cleared  when the alarm is acknowledged.   Other alarms may  be provided,  including  the indication  of  the failure of  the equipment  or  of a transmission to or from another signal box.   Fringe boxes may have the facility to change the transmitted description after its initial transmission. If this happens, the signalman will receive an  alarm  which  will  normally require acknowledgement.                      Use of Visual Display Units   Early train describer display systems provided panel displays only. The interface with the signalman was via an OCU as described  above.  More recent installations  incorporate VDUs  in addition to or in place of the conventional panel displays and OCU. The most common formats are as follows:-   Use of VDU in place of OCU. A standard VDU and keyboard may be considerably more versatile than the specialised OCU. As well as  being  used  for  input  and alarms, the signalman may be able to call up various area maps showing the train describer berths over a specific area of the   Use of VDU in place of panel indications. This arrangement is useful where a train describer is to be added to an existing installation. It avoids expensive  panel alterations and a large number of hard wired displays.  On  large panels,  it may  be more difficult to relate the train identities on the VDU  to the movement  of trains on the   This method of display can, of course, be used where the signalling is not operated from a control panel. Small train describer installations have been  successfully installed in signal boxes operated from lever frames. Additional VDUs for staff other than signalmen. Signal box supervisors, station staff and train announcers may all benefit from the information available in the train describer. Various area maps may be called up to provide relevant train running information. Input facilities are usually removed from such additional   Use of VDU for signalling control. To avoid the initial high cost of a panel and the difficulty of subsequent alterations, VDUs are increasingly being used as  a replacement for the custom built control panel. Depending on traffic  requirements, route setting information is entered via the keyboard, a mouse or a trackerball. The standard VDU graphics provide for the equivalent of all normal panel indications. Provided the display system has access to both signalling and  train  describer  data, both can be incorporated in a single   It is usual to provide both overview maps,  with  limited  signalling  data  displayed, and detailed maps showing complete indications for a  smaller  area. The signalman may have more than one VDU available.     1.             TRAIN DESCRIBER PLANNING AND DESIGN   The design of a train describer system will generally follow the main steps listed below.   Firstly, the general type of system must be decided. This will depend on the extent of area controlled, the density of train service,  other  interfaces  which  will  be required  and whether or not the existing signalling and/or control panel will be replaced at the same time. The incorporation of individual displays in an existing control panel can be  very  expensive. Usually, the best opportunity to install individual displays is when the  panel  is  fust constructed.   VDU based control and display systems can (at  least  in  theory)  accommodate  train describers within the signalman's display, provided they can  access  the  relevant  train describer data in a compatible format.   Assuming  the  area  of  coverage  is  to  be  a  complete  signal  box  area,  the   facilities  at   fringe boxes and links to adjacent boxes must be  determined.  Simple  fringe  boxes  may  only require simple input and output facilities. Others  may  require  some  stepping  within  their own area.   It is also possible to provide a distributed train describer system covering several smaller installations although the signalling inputs and the data links can prove quite complex. Having decided the type of system, the next step is to produce a block schematic. This will show all berths and all possible steps between them. It is a useful intermediate step in the  design process and enables the logic of the train describer to be discussed with  non­ engineering staff. Although the block schematic defines all berths and  steps,  we must still add  the stepping  logic. This is produced in tabular form, similar to signalling control tables, and  defines precisely the conditions required for steps to occur. The stepping conditions will  allow  the  designer  to define  the signalling  inputs  required.  It is important that the list of signalling inputs is complete.  These  will  have  to be allocated either to physical input circuits to the train describer equipment or, more usually in modem systems, to individual data bits in a TDM link, the data entering the train describer in serial form. It is also important to check that the signalling data is actually available at the correct physical input point!   We now return to our stepping tables to prepare the train  describer  data  ready  for compilation. In fact, it is now possible to prepare stepping tables in a format which  can  be  read directly by the train describer's data compiler.   The remaining task is now to define the available output formats. In the case of individual displays, each will require a physical output and a circuit from the equipment cubicle to the control panel. For VDU based systems, the maps will have to  be  drawn.  The  graphics package usually includes a library of standard characters to depict signals, various configurations of track layout elements and, of course, the train describer berths. The production of maps may be achieved by defining a text file or possibly by an interactive process.     4.1    Block Schematics   Block Schematics are used to show what facilities are to be provided by a train describer system. They are prepared from the signalling plan, generally agreed with the operating representatives, and used as a basis for stepping control tables etc. They frequently form a useful starting point for dealing with alleged faults, especially stepping faults.   The original BR train describer specification, BR800, defines the symbols used for train describer  schematics. The principal symbols are depicted on the following page. An example  is also shown of how the signalling plan can be converted to a block schematic.   The two ends of the station have heen drawn in different styles. One  in  the  form  of  a simplified  track  layout.  the   other   shuwing   ;m   indi  idu;il   line   for   each   step   or   pair   of   steps. BR Spcc.800 docs not define any preference between the two and practice may vary. The provision of a distinct line for each step does however permit the designer to check  that all steps have been listed on the stepping tables.   Each berth must be allocated a unique identity or address. This will be used by the train describer processor to manipulate the data. Usually the signal number  may  be  used  (often with leading zeros added).  Two  or  more  berths  associated  with  the same signal  will  have to be given different identities. This can normally be achieved by replacing the leading zero with an alphabetic character. It is also possible to  allocate  more  than  one  address  to  the same berth. This is useful on reversible lines to avoid confusing the signalman when  he is trying to enter a description. Train Describer Block Schematic Symbols Example of Block Schematic Production Track Layout Stepping Conditions   A step will always requires a TRIGGER, a change in  the  state  of  a signalling  input,  and may require one or more CONDITIONS, which must exist before the step is triggered.   Whether the step actually takes place however depends on the contents of the "FROM" and "TO" berths when the step is initiated. BRS00 uses the following rules:- Rule 1 covers the normal stepping function. Rule 2 assumes that if two trains JOill, the description of the second train will be carried forward. Alternatively, if a description  has been left in a berth for any reason, the next train will carry  its own  description  forward. This will also occur if either or both displays contain text (3, 9 & 10). If atrain carries a NDSD with it, rule 4 allows it to pick up a valid description  inserted  ahead. Rule 5 applies to the movement of a train without a description. NDSD is inserted and the alarm raised. Rules 6 & 7 ensure that a non described  train does  not overwrite  the existing contents of  the TO berth. 8a or 8b should be chosen as appropriate to determine whether or not text will step to an empty berth with the passage of a train.   4.3       The Basic Step lf signal 1 is an automatic signal, the step from berth 0001 to berth 0003 will take place when B Track Circuit becomes occupied. B T.C. occupied is therefore the TRIGGER  for the system. There is no condition.   TRIGGER        B Track Circuit Occupied.   STEP                000 I to OOOJ.   The system normally requires the trigger  to  be  present  for  one  second  before  the  step occurs, and to be absent for  three  seCDnds  before  the system  resets  and  allows  the step  to be repeated.     If signal 1 is a controlled signJI the description should only step if the  route  is set  and  the signal is clear. This information of route set or signal  clear  (usually  only  one  input  is  used, not both) will be available  from  the  interlocking.  These  will  be  included  in  the control  of the step as CONDITIONS, which must be present before the trigger  can  cause  the step  to occur.   CO:--:DITJON : 1 Route Set or 1 Signal Clear. TRJGGER                      : B Track Circuit occupied. STEP    : 0001 to 0003. Step Where Choice of Route Exists The train describer now requires  information  about  which  route  the  train  will  be  taking. This can be obtained in one of  several  ways  according  to  the availability  of  relay  contacts, the type of interlocking and/or the designer's preference.   CONDITION I (B) Route Set TRIGGER                        B Track Occupied STEP                   0001 to 0003   CONDITION : I Route Set (any route) and 101 N TRIGGER : B Track Occupied STEP         : 0001 to 0003     CONDITION : 1 Signal off and 101 N TRIGGER : B Track Occupied STEP         : 0001 to 0003   Where there are many routes from a signal in  a complex  track  layout  there  will  be  several sets of facing points. In this case it will generally be simpler to use a "route set" condition. If trailing points exist in the route they  are  not  included  in stepping  conditions  since  they give no choice. of route.                    Clear Out Conditions   When a train description is no longer  required,  (e.g.  train  entering  siding  or  train  leaving area of control) it is CLEARED OUT from the panel. The CLEAR OUT (CO) function is sometimes referred to as AUTOMATIC  CLEAR  OUT (ACO).  An  automatic clear  out can be considered to be a step from a berth to nowhere, and as such it  will  be  initiated  by conditions and a trigger.   Shunting Movements Usually, a train proceeding on a shunt class  route  into  sidings  will  have  its  description cleared out from the train  describer.  Thus  a  train  arriving  at  3 signal  and  proceeding  into the sidings will have its description cleared out. Similarly, a train arriving  at  4 signal  and setting back into the sidings using 301 signal will have its description cleared out.   There may be some movements where steps are provided for shunting movements. It is  important to obtain an accurate specification of  the  operators'  requirements  for  any  such steps. They can only be provided where the  train  movements  remain  within  the confines  of the signalled and track circuited lines. Overloading of Permissive Sections   A problem arises if the signalman sends another train into a pemuss1ve  section  (using ripple, shuttle or push-drag berths)  when  all  the available  T  D berths  are full since  there is nowhere for the incoming description to go. There are two common solutions to the problem. Either may be adopted for a particular situation. The choice will depend on the operating conditions - whether trains always continue in the same direction or reverse, whether trains are joined or divided etc. The train describer may either:-   Allow    the  incoming   description to overwrite the rearmost description in the permissive section.   or   Clear out the incoming description. Other applications for Clear Out conditions   Clear out conditions may also be applied in situations such as terminal platforms last berth on a panel ground frames TRAIN DESCRIBER STEPPING TABLES   There are many different styles of presentation. These have generally evolved  to  suit  the design of the interface with the signalling  equipment.  The  following  example  allows  the table to be followed easily  but  may  not  be representative of  current  practice,  either on  BR or elsewhere.   The choice of whether to use signal cleared or route set inputs depends on the information available which in tum will depend on the  type  of  interlocking  and  the form  of  the  data link. In the example below, steps from junction signals are conditional on signal cleared and point positions rather  than  identifying  routes  set. This  would  be  the  normal  situation  with a serial TDM link because the signalman's indications would not relate to individual routes. PROCESSING SYSTEMS   The fundamental function of a train describer is to  store  train  descriptions  in  individual berths and step those description from one berth to the next in response to trigger and conditions.   The earliest forms of berth to berth train describers were electromechanical using relay sets and uniselectors. Although a few examples remain in use today, this type of train describer is no longer manufactured.   During the mid 1960s electronically controlled train describers were introduced. These were designed as hard-wired logic systems, with solid state circuits performing the functions. With this type of train describer the relay sets of the electromechanical type are replaced with electronics - one "electronic store" per panel display. All stepping logic was wired into the system.   Computers were used initially to monitor the system and  provide  some  ancillary  facilities such as interrogation and train reporting.   Today, the non-safety nature of the train describer together with the low cost of the modest processing power required to perform the basic stepping and display functions has led to virtually all train describer installations being processor based.   Small systems can be produced to run on little more than a standard  desktop  computer although large control centres would  require  something  larger. This  has radically  changed  the economics of train  describer  provision.  Small  installations  with  perhaps  20-30  berths are now available at very low cost.   Each system will generally run on standard software  with  the  site  configuration  being defined in data for the stepping controls and display maps.   Computer based train describers possess  the following  advantages over previous generations of T.D.:   Standard hardware can be used for any  train  describer,  the  particular  local  berths and step arrangements can be specified in the software (program and/or data).   Modifications to the train describer caused by track layout alterations become easier since they only require software   The computer has the facility to  interrogate  all  berths,  looking  for a particular  train in response to a request from the signalman,  or  from  an  external  system.  This enables the Train Describer to become a source of train running information. 1.              DISPIAY SYSTEMS   Development   Various methods have been used for the display of train descriptions  on  the  Signalman's panel.   Early systems used a back projection system with separate lamps behind a stencil for each character, projected on to a screen. An alternative was an electromechanical moving stencil (generally restricted to a range of 10 characters per display). Both systems required frequent maintenance and would not meet the reliability requirements of a modern railway.   Miniature cathode ray tubes were the next development. Each  tube is mounted  in the panel  and fed with appropriate signals from character generation circuits. These have been very widely used, mainly as 2" x l" rectangular tubes,  but  occasionally  as  1"  diameter  round tubes.   Their main disadvantages were:-   They required a very high voltage supply (approx 2KV) which poses obvious safety problems in the confines of a signalling panel.   The power supply units used to derive the high voltage tended  to  produce  a lot of heat.   The C.R.T.'s fade with age and usage so it is virtually impossible to maintain all displays on a panel at a uniform brightness and character   The displays are difficult to mount and relatively   For the above reasons, light emitting diode displays soon became the most popular form of display where it was required to incorporate TD. displays in a  conventional  panel.  They usually consist of a 7 x 5 d1t matrix of LEDs for e:1ch uf the four TD. characters. The following advantages arc ascribed in the LED display:-   Use low voltage and have low power   Provide a display of constant uniform   Compact and easy to mount. 7.2 Visual Display Units (V.D.U.'s)   Visual display units (v.d.u's) were initially used in panel boxes  to  display  a  simplified pictorial "map" of the track layout and train descriptions, for use mainly by regulators, train announcers, etc. It was soon realised that such displays could be of great use outside the confines of the signal box operating floor.   It is now becoming more general practice with new train describer systems in existing signalboxes to use this as the only means of display.   The full benefits of VDUs can be provided where these form an overall control and display system for the train describer and the signalling system as a substitute for the conventional panel.     1.        SIGNALLING  INPUTS   The signalling functions that will be used to condition and trigger  steps  are  generally presented to the train describer in individual input circuits  (i.e. parallel data) where derived from a local relay interlocking.   Some form of relay interface is normally used between a relay interlocking and the train describer system for separation of power supplies.  For  a  duplicate  train  describer  system, two input circuits will therefore be required, generally fed over  contacts  of  the  same signalling input relay.   Where data is from a remote interlocking, a serial input sub-system driven by the  remote control system is provided. These are rarely duplicated.     2.             PERIPHERAL AND ANCILLIARY SYSTEMS   The modern train describer syslL'm conuins cxtcnsi·e data regarding the running of trains within the control area monitored. In addition it presents a useful display medium for train running dat from other sources.  The following  sub-systems  may  therefore  be found  as part of or communicating with the train describer in a modem signalling control centre:-   It will directly operate panel displays, visual display units and a technicians fault reporting printer.   It will receive inputs from O.C.U. key pads and signalling functions. It will be  required  to  communicate  with  adjacent  train  describers  and  with  fringe  boxes. It   may   provide    information    to   Automatic    Route   Setting   equipment    and  Passenger Information  Systems.  These  sub-systems  will  generally  be  driven  by  their  own computers rather than form part of the train describer system. Some of  these  other  sub-systems  are briefly described below.             Automatic Train Reporting (A.T.R.)   It has been customary to provide a record of train running for future reference in case it is necessary that incidents need investigation. This was originally done by the signalman, with assistance in busy signal boxes. Automatic Train Reporting uses the data available from the train describer to record the times of trains passing certain signals and relieves the  signalman of the responsibility.   A.T.R. information may be provided in  a transient  form  using  a VDU  or as a hard  copy  using a printer. It can be made available to Signalmen, Regulators,  Control  and  other  operating supervisors according to their needs.   Whereas earlier A.T.R. facilities were controlled by the train  describer  directly,  it  is  now more common to provide a separate computer  sytem  for  this,  and  other  information  facilities.   The basic format for A.T.R. is:-  Automatic Train Reporting By Exception (A.T.R.E.)   In a busy area an A.T.R. system  will  produce  an  extensive  record  of  train  movements during a normal day. However, it is of little interest to record the trains that run to time. Regulators and, timetable planners want to know which trains run late so that they can make short and long term adjustments respectively.   If the A.T.R. computer can be provided with the expected passing times for trains, it can compare actual passing times and only log those which are not in agreement. The Master Timetable System computer provides such information though it must be up dated daily to account for irregular train movements.   A realistic tolerance before or after the booked time must be allowed in  the  comparison between booked and actual times before a report is made. Two minutes is normal. Thus the report is considerably reduced in length and contains only those trains that need attention.                      Train Running System on T.O.P.S. (f.R.U.S.T.)   BR operates a system called TOPS (Total Operations Processing System) to monitor rolling stock movements. When first introduced, all stock movement data was  input  manually. Provided the consist of a particular train is known, the location  of  each  vehicle  can  be updated by way of train movements information.   Train running information from the train describer is passed to the TOPS network for comparison with the timings  held  by TOPS, and so that  the train's location  within TOPS can be kept up-to-date. The information is available  immediately  at  teleprinter  or  TOPS terminals, and is stored to allow for statistical analysis of train performance. Automatic Speed Measurement   The A.T.R.. system can  be  used  to register  a  train  speed  in  addition  to its passing  time  at a signal. It is achieved by  measuring  the time taken from stepping  the train description  into the berth to stepping it out again. The distance between triggering points is known and the system divides it by the time to calculate the average  speed  of  the  train  over  one section. This speed, and the maximum permitted speed, will be displayed as part of the passing time report.   e.g.  11.00        AH0120        1T15        70MPH, 80MPH    Hot Axle Box Reports   Hot Axle Box Detector Information has usually been presented in separate display units mounted adjacent to the control panel. The necessary information can be input to the A.T.R. system and displayed either on a VDU screen or as a report associated with the next signal passed.   Passenger Information Systems   Platform indicators and passenger announcements may be operated automatically. The A.T.R. or a dedicated sytem will be used to recognise a train description and display the  relevant information on the correct platforms as the  train  approaches  stations.  The information format for the public  varies  widely  and,  therefore,  the  system  programs  must be designed around the needs of the area served.   Announcements are triggered in a similar way as the train approaches stations.   It should be remembered however that the train describer  information  is  not  always  sufficient for full passenger information to be given in adequate  time. The  main  problems are:-   Frequent changes of platform, often at short   Routes not set early enough to provide passengers with sufficient   Control area does not cover a large enough approach area. Data has then to be obtained from adjacent systems.                  Automatic Route Setting   A signalman is required to set routes such that trains run to schedule. He does this  by comparing the actual position of a train with its  scheduled  position  and  may  amend  his action for several trains should one be 0!-1t of schedule. A computer based Automatic Route Setting sytem has been developed which takes current train  positions  from  the  train  describer, scheduled positions from the Master Timetable  system  and  then  optimises  the route setting to most effectively attempt to meet the scheduled requirements.   The programme for this requires to account for connections that should be  kept  and conflictions that may occur.     Train Radio   A secure radio link between the signalman and each train driver is a requirement  for Driver Only Operation of passenger trains. This requires that each locomotive  is provided  with a radio, uniquely coded with that loco's  stock  number  (eg.  47.001).  The  train  describer  is used to correlate the train description with the locomotive  stock  number,  so  that  the signalman can call trains by referring to their description on the panel.   At the start of each journey the driver sets up the signal position of  his  train  on  a  thumb-wheel switch, which is encoded and transmitted  along  with  the loco  stock  number. The train describer is then interrogated for the description at that address. Any calls the signalman makes to that description will then automatically be converted to a call to  the relevant locomotive, and similarly calls from the driver  will  be  identified  by  the  relevant train description.   At the end of the journey the train describer will automatically  remove  the  correlation between the loco number and train description.    

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-
Posted 98 days Ago

Train braking performance determination

During their historical development, the railways in Europe have adopted their own technical standards and operating rules according to national requirements. As a result, the European railways use different train control systems (INDUSI, KVB, LZB, TVM, ATB etc.) and have different warning distances (400m to 6,000m). This situation constitutes a serious obstacle for interoperable high speed and conventional trains. To overcome this obstacle, the ERTMS/ETCS standard European train control system was developed.   The ‘Specifications for the braking deceleration of trains for ETCS/ERTMS’ Project aims to ensure the interoperability as defined in Directives 96/48/EC and 2001/16/EC with respect to interoperability of the trans-European high speed and conventional rail system. Braked weight percentage Currently, for virtually all trains in Europe operating at up to 200km/h, the only information available for the quantitative description of the braking performance is the braked weight percentage. The braked weight is usually determined empirically on the basis of stopping distances obtained in tests (according to UIC Leaflet 544-1). In 1938, the French engineer Pedeluc from PLM (Chemins de Fer Paris-Lyon-Mediterranée) performed a series of braking tests using 15 identical passenger coaches by applying an emergency brake from different initial speeds. He defined the mass of every vehicle as its ‘braked weight’ and called the quotient of ‘braked weight’ and vehicle mass the ‘braked weight percentage’ (or l-value).   As a result that (standard) test train had a nominal λ-value of 100%. Different train masses were simulated by simply switching off the brakes of one or more vehicles on the assumption that the λ-value is constant. As the total mass of the train remains unchanged, but the mass of the vehicles contributing to the braking performance decreases, it was possible to generate graphs of stopping distances for each initial speed. On the basis of the test results he developed graphs for each initial speed to relate the three parameters of stopping distance, initial speed and λ-value with each other. This allowed the evaluation of the braking performance of a given train on the basis of its stopping distance.   In a double logarithmical scale these graphs appear as straight lines allowing an easy assessment of any train’s braking performance (see  Figure 1 ). A similar method called slip-brake tests was used to create graphs for individual vehicles (see  Figure 2 ). To obtain a vehicle’s (or a train’s) ‘braked mass’ it is sufficient to multiply the λ-value find as a test result with the vehicle (train) mass. The established value is independent for all initial speeds (however this is only correct for cast iron friction material).     Consequently, brake tables were developed by each railway defining the required λ as a function of the braking (and consequently signalling) distance to operate a train at a given speed or vice-versa to determine the maximum speed for a given λ-value. This system has been adopted by nearly all European railways as the calculation of the braking performance was easy, by simply adding vehicles’ masses and vehicles’ braked weight and dividing the two values. It is an effective way of expressing the ability of any train to stop over a certain distance when travelling at a given initial speed. However, this parameter does not provide any information on the actual deceleration characteristics, which can vary widely depending on the braking equipment. The above characteristics indicate that it will still be important in the future to ensure that the braking performance of trains can be described predominantly in terms of the braked weight percentage. Train braking with conventional signalling In the case of conventional signalling, where information is provided by lateral trackside signals, in theory there is one point at the beginning (warning signal) and one point at the end (stop signal) which must be observed. Between these two points the profile of the train’s deceleration is generally not subject to control and depends on the type of brake with which the train is equipped. An automatic speed supervision system like ERTMS/ETCS (see  Figure 3 ) requires knowledge of the braking performance of a train in the form of its instantaneous deceleration as a function of speed. Clearly, a solution therefore had to be found to use the braked-weight percentage, l (and, if necessary, other additional information), to derive a reliable deceleration profile.   Methods for calculating instantaneous braking decelerations As a consequence, a new method to describe the braking performance of all train categories running in an ERTMS/ETCS environment had to be developed. Emergency braking curves are necessary for safety. As timetables are never based on emergency brake applications, but on service braking, line capacity is limited by service braking curves. To achieve an increase in line capacity, a definition of standardised service braking curves will be essential for the performance of the system. Three different mathematical models were developed to translate the l-value into an instantaneous deceleration function. These models were all based on different theories: The FS model has a theoretical basis in certain assumptions on the properties of brake systems. The NS model derives decelerations from the known speed-dependency of specific types of brakes. The DB model is an empirical adaptation of an old UIC method used in the LZB signalling system. Unlike the FS and NS models, it does not give a result in the form of a step function for instantaneous decelerations versus speed. All three models calculate the instantaneous braking decelerations from known train data, including the braked weight percentage. To determine which model would best suit the requirements, results of actual brake tests, recently performed by different railways to assess the braking performance of their rolling stock, were collected. These results were compared with the values each model would calculate. In order to assess the quality of the models, the predictions they yield (for decelerations and braking distances) have been compared with a large number of experimental results (approximately 150 sets of test results). After ten months of intensive work, it was determined that the conversion model developed by FS delivered the most consistent and accurate results, calculating safe distances for all train types for which test results where available. Due to the lack of existing test results of some important types of train combinations (e.g. freight trains with cast-iron brake blocks or passenger trains with cast-iron block brakes) it was not possible to evaluate the model for all train types. In order to test the suitability of the model in everyday service, it was necessary to perform tests on ETCS equipped lines. It was therefore decided to perform dedicated tests in a second phase with such trains to close this knowledge gap. Although the basic suitability of the conversion model for use in ETCS was established, the existing model could not yet be recommended for immediate inclusion in the ETCS-SRS or indeed in the TSI for the time being. There are two main reasons for this: The braking trials on the ETCS test lines could not be carried out within the timescale planned. Various railways, for example SBB and DB, have stated they are not in a position to use the current model on Level 1 lines, as it generates stopping distances for certain train types that are excessive and not compatible with existing signal spacing. The criticism of the safety margins inherent in the model, which can be seen as the principal cause of the incompatibility on Level 1 lines, is fully justified. Unfortunately, it came at a point when the Working Party no longer had sufficient time available to conduct a comprehensive optimisation of the model. It is also worth noting here that the λ-value that is currently used exclusively to describe the braking power was derived from the deceleration characteristics of grey cast-iron brake blocks. The superior performance of disc brakes It had already been established long ago that vehicles fitted with disc brakes have considerably improved braking performance and therefore greater safety margins at higher speeds than those with cast-iron tread brakes, in other words they achieve shorter braking distances than those specified in UIC Leaflet 544-1. All that could be done with the model to reconcile this fact (with the aim of arriving at a standard conversion model that is not linked to the type of braking equipment used) was for the model to deliver realistic deceleration values for tread-braked trains fitted with cast-iron blocks. Consequently, the deceleration values provided by the model were too conservative for disc-braked vehicles and for vehicles with additional brake components like electro-pneumatic brakes, brake-pipe emptying accelerators or magnetic track brakes. The solution to overcome these constraints was to improve the current model by modifying some of its parameters and to re-enter the values from test results to assess the impact on the braking distance. In a final attempt, the WP evaluated different parameter modifications on the basic model for all train types, which meant a cross-check of more than 250 test results for each modification of the model. This work was not in vain, as it turned out to be possible to generate an advanced model still applicable for all train types. The only additional parameter to be introduced is to distinguish between brake position P for passenger trains and G for freight trains, taking into account the different brake development times of the distributor valve for the two positions (3-5 seconds to achieve 95% of the maximum cylinder pressure in P compared to 18-30 seconds in brake position G). As an additional advantage the model is widely compatible with the existing brake tables used by the railways, so no additional measures will be necessary to run a train on ETCS-equipped lines as well as on a classic line with lineside signalling. There are still open points to be dealt with, including testing the advanced model for service brake applications. Such testing can only be performed on an ETCS pilot line with a locomotive fitted with the original onboard equipment. Such tests are planned for 2005 at ÖBB. Another major issue still to be resolved is the development of standardised safety margins for ETCS-lines to avoid changing the algorithm to calculate the guaranteed deceleration at network borders. It is to be anticipated that with the introduction of this standardised conversion model a considerable step towards the interoperability on ETCS-lines is achieved that is intended to contribute to the success of ETCS/ERTMS.

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Deepu Dharmarajan -
Posted 92 days Ago

TRACK CIRCUITS

  CONTENTS 1. Introduction 2. Principles of Operation 3.Practical Considerations 4. Insulated Block Joints 5. Track Circuit Types 6. D.C Track Circuits 7.  AC Track Circuits 8. Jointless Track Circuits 9. Impulsing Track Circuits 10. Rail Circuits and Overlay Track Circuits 11. Shunt Assisters 12. Alternatives to Track Circuits 1. INTRODUCTION The track circuit is an important component of most modem signalling systems. Its main purpose is to prove that a particular section of track is clear of trains or vehicles. The first track circuits were used only as indication devices, to remind the signalman of the presence of a train, for example standing at the home signal or on a section of track which might be obscured from his view. The signalman was still responsible for operation of the signals. Later they were used to lock facing points, as a replacement for fouling bars. In this role, the track circuit directly controlled the operation of the point lever through an electric lock. Track circuits are now used to control the signalling directly, by preventing a signal being cleared if the track ahead of the signal is occupied by a train, by holding the route after a train has passed the signal, and to lock a set of points if a train is standing on them. It is important to remember that a track circuit is designed primarily  to detect the absence of a train and not its presence. There are however many signalling controls which require track circuits to be occupied (e.g. release of overlap after a train has come to a stand) and the track circuit must therefore be reliable enough to be used for this purpose also. A rail circuit is the reverse of a track-circuit, and is used to prove the presence (not absence) of a train at a particular place. Rail-circuits are not as commonly used as track circuits. To understand the basic principles of track circuit operation, the easiest type of track circuit to study is the simple, battery-fed D.C. track circuit. Many other types have been developed to cover particular situations such as electrified areas, jointless track etc. These will be studied later. Most of the principles of the simple D.C. track apply equally to other types, although the configuration of the equipment may differ. 2. PRINCIPLES OF OPERATION A track circuit  employs  the rails as a transmission  line. The electrical  signal, in this case a d.c. voltage, is applied to one end. At the other end a relay is connected  across the rails as a receiver of the voltage applied at the opposite end. The physical limits of the track circuit must be defined by insulated rail joints to prevent interference between adjacent track circuits or isolate the track circuit from sections of line where track circuits are not required. When there is no train on the track circuit, the relay will be energised via the full length of each rail. When a train stands on the track circuit the wheels and axles create a short circuit between the rails. The current takes the lower resistance path via the train and the relay becomes de-energised. All track circuits depend for their operation on the wheels of a train forming a low resistance path between the rails. SIMPLE D.C. TRACK CIRCUIT Track Circuit Clear (Relay Energised) Track Circuit Occupied (Relay De-energised) The relay is therefore energised to prove that there is no train on the track, and de-energised when there is a train on the track. The relay will also de-energise if: a. A rail connection becomes detached b. The power supply fails c. A rail to rail bond is broken or becomes detached d. A rail breaks The circuit is therefore designed to be "fail-safe", as under most fault conditions it indicates that the track is occupied. This may not be totally true in practice, as will be seen later. 3. PRACTICAL CONSIDERATIONS A practical track circuit differs significantly from the theoretical example shown earlier. The two major difficulties are:- The rails are not perfectly insulated from each other. Current "leaks" from one rail to the other, through the rail fixings, the sleepers and the ballast. This rail-to-rail resistance is known as "ballast resistance". It can vary 300 metres of standard gauge (1435mm) track may have a ballast resistance as high as 50 ohms for well ballasted, dry track, or as low as 0.5 ohm with waterlogged or dirty ballast. The presence of the wheels on the rails may not provide a perfect short circuit. The axles may have a small resistance, but the main problem is the contact between wheel and rail. This may be due to rust or other causes. Although the rails also have a resistance, this is normally insignificant compared to the above two problems. Rails will also have an inductance. This will be significant when  dealing with some types of a.c. track circuit. 3.1 Ballast Resistance This is a factor over which the signal engineer usually has very little control. It will vary according to the type of ballast, its condition, the type of sleepers, the method of fixing of rails to sleepers, track drainage and, most importantly, the weather conditions. These will all vary in time during the life of the track circuit. Some factors will change slowly, such as the condition of the ballast. Others, like the amount of moisture in the ground, can vary rapidly. The design of the track circuit equipment must be able to compensate for the long term changes by some means of manual or automatic adjustment. The short term changes due to weather must be contained within the normal operation range of the track circuit. If not, failures (either right side or wrong side) will result. The inclusion of a series resistance at the feed end is an essential feature of most track circuits to avoid excessive power consumption when the track is occupied. It also permits components of a lower current rating to be used. This will also make the track circuit more difficult to set up for reliable operation. As ballast resistance decreases, the current drawn will increase, the voltage drop across the feed resistor will increase and the relay voltage will be correspondingly reduced. Too large a reduction in ballast resistance could cause a right side failure of the track circuit. 3.2 Train Shunt This can also vary over quite a large range. It is dependent on the electrical contact between wheels and rails. For most track circuits we require the first axle of the train to shunt the track. Otherwise the track circuit could not be used for proof of clearance at points and crossings. A commonly used and accepted value of train shunt for testing is 0.5 ohms. Where the trains are infrequent, a film of rust can quickly build up on the rail, causing poor contact and potential wrong-side failure. This may be a particular problem where there is no service over a weekend and traffic resumes on a Monday morning. Not surprisingly, the problems increase where short trains of lightweight vehicles are in use. This is due both to the reduced pressure on the contact area of the wheel and the reduced cleaning effect on the rails as each train passes. With the introduction of modem rolling stock new problems have appeared. The disappearance of wheel rim brake shoes in favour of disc brakes, together with the improved riding qualities of new suspension systems, both decrease the cleaning effect on the contact surfaces. A reliable train shunt is therefore more difficult to obtain. To overcome the problem of a poor train shunt resistance, a stainless steel strip may be  welded to the top of the rail or a treadle may be cut into one of the track circuit connections. Both measures ensure that the track relay de-energises as soon as the first axle runs on to the track circuit. The relay should then remain down as more axles of the train run on to the track and lower the value of the train shunt. Such features are particularly vital where track circuits control automatic level crossings and accurate timing is vital. 3.3 Relay Characteristics There are two different values for train shunt. The drop shunt is the largest value of train resistance that will cause the track relay to release. The pick-up shunt is the smallest value of train shunt which will allow the relay to operate. Due to the method of operation of all relays, the drop shunt will always  be the lower  of  the two values. As the energisation of the relay closes the air gap in the relay's magnetic circuit, the reluctance of the magnetic circuit reduces. The relay therefore requires less current to hold it in the energised position than to pick it up initially. Conversely, it will require a significant reduction in this current to release again. Whenever track circuits are tested the drop shunt should always be observed and recorded. Track relays are designed specially to minimise the difference between the pick-up and hold-up currents. Nevertheless, a hold-up current of only 0.5 to 0.7 times the pick-up current is typical. Another problem is the time of release of the track relay. With both relay and rail being inductive, a loop will be formed by the rails, the train and the relay. The back-emf caused by the reduction in current as the train shunts the track will tend to cause a circulating current in this loop which will delay the release of the relay. Short, fast moving trains could momentarily be undetected as they pass from one track circuit to the next. This has serious implications for the release of route locking and the holding of points. If the type of track relay used is not inherently slow to pick up, a repeater relay which is slow to pick must be used in all vital signalling controls. 3.4 Maximum Length The combination of the above factors will generally limit the maximum length of any type of track circuit. The longer the track-circuit, the lower will be its ballast resistance, and so the attainable voltage at the relay will be lower, as more current leaks between the rails. Eventually, a length is reached at which it is no longer possible to operate the track relay. This maximum length will be lower for lower values of ballast resistance. The maximum length must therefore be. based on worst case ballast conditions. A track circuit adjusted for minimum ballast resistance (e.g. during wet weather) will have a relay voltage higher than the minimum pick-up voltage when ballast conditions improve. Adjustment must be such that the track relay does not remain energised when a train of maximum value of drop shunt occupies the track circuit. This requirement also effectively imposes a maximum length on the track circuit. Maximum length will depend on the type of track circuit, the components used and the conditions under which it operates. Most track circuits have maximum lengths in the 600  to 2000 metres range. 3.5 Minimum Length For short track circuits, adjustment for reliable operation does not usually present any problem. But if a track circuit is too short, a vehicle with a long wheel-base may stand completely over the track circuit without it being detected. The minimum length of a track circuit is set by the vehicle with the longest wheel-base.  For  bogie vehicles,  this distance is measured between the inner axles of each bogie. In most cases, this minimum distance is more likely to be determined by bogie vehicles than by two axle vehicles. 3.6 Staggered Block Joints Ideally, insulated joints are installed in pairs opposite each other. In practice, it  is frequently difficult to ensure that they are exactly opposite, such as within pointwork. Joints may be provided to separate adjacent track circuits or to allow a polarity change within a track circuit. Where joints are not exactly opposite, they are said to be staggered. There. is a short length of track where both rails are of the same polarity. In this area, an axle will not be detected. If the stagger is too great, a bogie or even a complete vehicle may remain undetected. The maximum stagger is determined by the vehicle with the shortest wheelbase. Although it is still possible to "lose" one axle, the other axle will be detected, Care must be taken to correctly position joints for clearance purposes to take account of this problem. Staggered joints must not be used to define critical clearance points. If two sets of staggered joints are too close together, both axles of a short, four wheel vehicle could be undetected at the same time. A set of joints which is not staggered but defines the end of the track circuit could also fail to detect a vehicle if too close  to  staggered joints. Therefore, unless joints are exactly opposite, they must always be separated by the length of the longest vehicle. Track Circuit Minimum Length Staggered Joints - Maximum Stagger Joints in Close Proximity - Loss of Train Detection 3.7 Equipment Details To ensure effective operation equipment types and values must be carefully chosen. Whether a.c. or d.c., power supplies are normally of low voltage. This is desirable as the presence of a train presents a short circuit to the feed set or transmitter. Similarly, there is usually a series impedance in the transmitter/feed set to limit output current, avoid damaging circuit components and prevent excessive power consumption. Where power supplies are derived from an ac mains supply, each track feed will  normally have its own supply to prevent track circuit signals feeding through between adjacent track circuits via the power supply. Where a feed resistor is provided it is often adjustable to deal with different track circuit characteristics. When a track circuit is set up, account must be taken of the weather & ground conditions at the time. An incorrectly adjusted track circuit may- either fail to detect all trains under conditions of high ballast resistance (dry weather) or fail to energise the relay with no train present if the ballast resistance is extremely low (wet weather). Many audio frequency track circuits are designed to be adjusted by varying the gain of the receiver. This makes setting up easier and also permits track circuits to be centre fed where required. Frequencies for a.c. track circuits are normally within the audio frequency range. Higher frequencies are increasingly attenuated by the inductive impedance of the rail. This would make the maximum length impractically low. Mains frequency may be used in non-electrified or d.c. traction areas (with suitable precautions). Due to the low power used, relays are normally of low coil resistance (typically  4  to 20 ohms). For the same reason, relays may only be provided with two front contacts. Electrical continuity of both rails is essential. Any rails not welded or bonded for traction return must be bonded by the signal engineer.  Bolted fishplates do not in practice provide  a reliable electrical connection. For rails which carry track circuit currents only, an adequate connection can be made by using two galvanised steel bonds pinned at each end. Welded or brazed connections may also be used. Where electric traction is used, the return traction current will be many times greater than the track circuit currents. The bonds must be much heavier and will often be provided by the electrification engineer. Connections from the rail to lineside equipment should be firmly secured to the rails and employ flexible cable to counteract the effects of vibration. 4. INSULATED BLOCK JOINTS Except for jointless track circuits on plain line, insulated block joints are used to define the limits of a track-circuit. It is important that the failure of an insulated joint does not cause  a wrong side failure. Joints may be provided in one rail only or in both rails. This will usually be decided by requirements for electric traction return. With joints in both rails, if a single block joint fails, both track circuits should continue to work satisfactorily. If both block joints of a pair fail, there is a danger of a relay being falsely energised from the adjacent track-feed. To avoid this problem, the polarities of the rails on either side of every insulated joint must opposite. If both block joints of a pair fail, the two feeds will short-circuit each other, and both relays will de-energise. In the vicinity of points and crossings, it may be necessary for sections of one rail to be common to two track circuits. Failure of any common blockjoint should cause both track circuits to show occupied. In electrified territory, joints may be in one rail only. Maintenance of opposite polarities at every blockjoint is essential as only one joint separates the track circuits. Track Circuit Polarities Double Rail (Joints in Both Rails) Single Rail (Joints in One Rail Only - Other Rail Continuous) 5. TRACK-CIRCUIT TYPES Many different types of track circuit are available, designed to be used in particular applications such as electrified lines, continuous welded rail, etc. A list of the main types of track circuit is given below. As track circuit development continues, further types will no doubt become available. It is not necessarily an exhaustive list. 5.1 D.C. Track Circuits D.C. track circuits have the advantage that they can be simply fed from a battery, either as the main or a standby supply. Installation, setting up and subsequent maintenance is straightforward. They have the disadvantage that they are unsuitable where d.c. traction systems are in use. Battery - fed (primary cell or trickle charged) Non AC - immune AC - immune Coded "Swept" 5.2 Fixed Frequency A.C. Track Circuits All track circuits in this category work from a mains power supply at the same frequency. Mains frequency (e.g. 50Hz) vane relay (single-rail or double-rail) Traction immune, vane relay (operates at a different frequency to the traction supply for immunity from a.c. and d.c. traction currents) A.C./D.C (a.c. track circuit employing a d.c. relay) various types 5.3 Multiple Frequency A.C. Track Circuits Track circuits in this category are normally designed and installed so that adjacent track circuits operate at different frequencies. In most cases they may be used with@ut insulated blockjoints. Reed Aster CSEE UM71 ML Tl-21 Westinghouse FS2500 5.4 Impulse Track Circuits These track circuits employ short pulses at a relatively high voltage to overcome contact resistance at the rail while keeping power consumption at a low level. Jeumont Lucas Although in some cases proprietary or manufacturer's names have been shown above, similar equipment may be available from other manufacturers. 5.5 Functions Other Than Train Detection As the rails are a continuous transmission line from one end of the track circuit to the other, it is possible to use the track circuit for more than simple train detection. To economise on lineside circuits, additional controls can be added into the feed end of the track circuit, typically in automatic sections. For example, the track feed can include proof that the signal ahead of it has returned to normal after the passage of the previous train and/or proof that the signal is alight. The disadvantage of this type of circuit is that it complicates fault finding and may give the signalman misleading track circuit indications. A more recent application is the coded track circuit. The track circuit current is interrupted at different intervals or modulated at different frequencies to convey information to the train, usually for automatic train control (ATC). The current passing from one rail, through the wheels and returning to the other rail is detected by one or two receivers on the front of the train. Different signals may correspond to different commands to the on-board ATC equipment. To operate correctly, the track feed must always be at the end of the track at which the train leaves. Otherwise the train will shunt its own track circuit signal and the receiver on the train will see no signal·current in the rails. This is not a problem where trains run in one direction only . On bi-directional lines, this means that the track feed and relay circuits must be capable of being switched from one end to the other or two sets of track circuit equipment must be provided for each bi-directional track. Coded track circuits also create complications in train detection at the relay or receiver end. The simplest approach is to build in some form of delay longer than the code pulses to maintain the relay energised. The relay/receiver could be set up to recognise only the available set of codes, any invalid code causing the track circuit to show occupied. The third option is to compare the signals at the feed and relay ends. Coded Track Circuit - Operating Principle 6. D.C. TRACK CIRCUITS In its simplest form the d.c. track circuit is as described in the earlier sections. Power was originally derived from one or more primary cells which would be replaced as necessary during routine maintenance. Where a lineside signalling power supply is available (normally a.c.) this will be used to operate the track circuit, either via a transformer & rectifier or by trickle charging a battery of secondary cells. In more recent years, solar power has become a viable power source in areas where the climate is suitable. The choice of feed arrangements will depend upon the reliability of the main power supply. If there are standby arrangements for the supply itself, a battery is not normally used. In ac electrified areas, d.c. track circuits may be used but with special a.c. immune feed sets and relays. Immunity to the a.c. traction currents must ensure that the feed circuit cannot be damaged by traction currents and the relay will not pick or be damaged  by a.c. A combination of fusing, series chokes and a.c. immune relays will normally provide acceptable protection. Purpose built a.c. immune feed sets are available. Obviously, d.c. track circuits cannot be employed where d.c. traction is in use as there is  no satisfactory means of preventing traction currents from passing through the relay. 7. A.C. TRACK CIRCUITS A.C. track circuits, normally at mains frequency, can be used in areas where the presence of high levels of d.c. would prevent the use of d.c. track circuits. This is most commonly due to the use of d.c. traction, but may also be due to the presence of certain chemicals in the ground, causing a voltage to appear across the rails due to electrolytic effects. It is also possible to use a.c. track circuits in non-electrified areas. In place of a d.c. feed, an a.c. source is used. A two element a.c. vane relay is usually used in places of the d.c. relay. A.C. vane relays cannot be energised by a d.c. voltage. The vane relay is operated by two windings, both of which must be energised simultaneously. One winding is fed locally, the other through the rails. The two relay feeds are then approximately 90 0 out of phase. If either feed is absent, or the phase angle between them  is wrong, the relay will not energise. To isolate the track circuit equipment from the effects of very large traction currents, it is normal to connect the track circuit equipment to the rails via either a transformer or a series capacitor. If a capacitor is used in the feed end, this can be variable to perform the function of the feed resistor in a d.c. track circuit. Fuse protection is also desirable. One point to be borne in mind when designing locations for a.c. track circuits is that the supplies for the two windings must come from the same source. Signalling power supplies are single phase. This is normally derived from a 3-phase supply and adjacent power feeders may not be fed off the same phase. As the relay is phase sensitive, failure to  observe this rule could lead to incorrect operation and/or adjustment of the track circuit. If d.c. traction is used, voltages are usually low and currents are therefore extremely high. To provide an acceptable low resistance return current path to the substation, both rails may be needed for traction return. Two basic configurations of A.C. track circuit are required, Double Rail and Single Rail. This refers to the traction current, not the track circuit current. Phase must be alternated between adjacent tracks for the same reason as polarity is staggered with D.C. track circuits (i.e. failure of blockjoints, connecting two supplies in antiphase will produce an effective voltage which is insufficient to energise either of the relays). 7.1 Double  Rail Track circuits    With low voltage d.c. traction systems, currents of 3,000A or more are commonplace. If current is to pass along both rails, special arrangements are made at track circuit joints to pass d.c. but not a.c. The arrangements include the use of an "impedance bond", which presents a low impedance to d.c., but a high impedance to a.c. by virtue of its two coils being wound in opposite directions. Often the impedance bond is "resonated". The impedance bond is provided with a secondary winding of a larger number of turns than the primary windings. This is connected in circuit with a capacitor, giving a band-stop filter effect at the track circuit frequency to further increase the a.c. impedance. The impedance bond is effectively a centre tapped inductor. The d.c. traction currents in each half of the winding set up opposing fluxes in the magnetic core which cancel each other out. To the a.c. track circuit current, the impedance bond presents a rail to rail impedance which is high compared with the ballast resistance. Failure of a blockjoint or the disconnection of a traction return bond between rails may unbalance the d.c. currents in the impedance  bond  sufficiently  to saturate  the magnetic  core of the impedance bond and cause the track circuit to fail. In addition to the extremities of a track circuit, impedance bonds will also be found:- Where it is necessary to bond the traction return between adjacent tracks (to provide a lower resistance path). Depending on the power supply arrangements, this is usually necessary at approximately 0.9 km (BR third rail 750v) to 1.5 km (SRA 1500v overhead) Immediately adjacent to substations for connection to the traction supply. 7.2 Single Rail Track Circuits If traction currents can be kept low enough it may not be necessary to use double rail track circuits throughout an electrified line. Through points and crossings the use of double rail track circuits may not always be possible or desirable. Single Rail  track  circuits will then be used. The length of single rail track circuits through pointwork should be kept as low as possible where D.C. traction is in use as the impedance of the traction return path will be increased. A.C. Double Rail Track Circuits Typical SRA BR Capacitor Fed Typical Single Rail Track Circuit Double Rail Track Circuits with Additional Impedance Bonds Dual Electrified Lines In a few cases, more than one electrification system is in use on the same line (e.g. 750v d.c. third rail and  25kv  a.c. overhead)  or lines  on  different  systems run in close  proximity. If mains frequency a.c. track circuits are used, they will not be immune to the a.c. traction current. Similarly, d.c track circuits  cannot  be  used  either. It is however possible  to operate at a supply frequency different to the  traction supply (and any harmonics),  Immunity will then be provided against both traction systems. For a 50Hz traction supply, frequencies  of 83.3Hz and 125 Hz have been used. A separate signalling supply at the required frequency can be provided  and  distributed  along  the lineside. Alternatively, the supply for  each  track  feed  can  be produced  at  each  location by an inverter. Filters must be incorporated in both feed and relay ends of the track circuit. Impedance bonds on double rail track circuits must  be  resonated  at  the  track  circuit  frequency (not 50Hz). This will require a different (usually lower) value for the resonating capacitor. The impedance bond will then appear as a much lower impedance at 50Hz. This arrangement is particularly useful where the traction system is to be converted from d.c. to a.c. where much of the existing track circuit equipment can be retained and track circuit conversion can be undertaken in advance of the traction changeover. The use of such track circuits is now falling in popularity as a number of dual traction immune audio frequency track circuits are available. 7.4 A.C/D.C. Track Circuits The simple d.c. track circuit has an inherent slow-to-release characteristic, because  the track relay has a high inductance, and a low resistance. The d.c. track relay is however much simpler and cheaper to manufacture than the a.c. vane relay. The high inductance delays the release of the relay. This is an unwanted characteristic for a track  circuit, because it delays detection of fast moving vehicles and could leave a short train or vehicle undetected as it passes from one track circuit to the next. This obviously has serious implications on the safe operation of route locking. If a relay with higher resistance, and lower inductance is used, the relay releases quicker. However, a relay with higher resistance requires a higher operating voltage. An equivalent effect can be obtained by feeding a d.c. track relay through a step up transformer rectifier arrangement. One example of this is the B.R. "Quick Release" track circuit. The voltage on the rails is a.c. The resistance in series with the track relay increases the value of resistance in the shunted circuit, and reduces  the release time.  The fact that the relay end has a high resistance also makes it possible to locate the relay some distance from the end of the track circuit (useful in restricted access situations such as tunnels). This type of track circuit is not traction immune. It is also not frequency selective, and will operate at frequencies other than 50Hz, which causes problems when used adjacent to audio-frequency jointless track circuits. 8. JOINTLESS TRACK CIRCUITS Insulated block joints are an additional cost in modem welded rail. Although the strength and reliability of insulated joints has generally reached a high level, most permanent way engineers prefer to avoid insulated joints wherever possible. Jointless track circuits have been developed to operate without insulated block joints. Various arrangements of electrical components are used to terminate the track circuits, and define their limits, electrically. All jointless track circuits employ an a.c. signal. A transmitter at a specified frequency is connected to one end of the track circuit. At the opposite end, a receiver is connected  which will only respond to the signal produced by its own transmitter. The receiver  will  normally be used to operate a relay which interfaces with the signalling system in the normal manner. The insulated joint normally takes the form of a filter which will prevent signals of the track circuit's operating frequency from penetrating more than a short distance beyond  the  transmitter or receiver and ensure that the track circuit is not operated by signals from adjacent track circuits. Adjacent track circuits must therefore operate at different frequencies. The frequencies used must also be immune from mains interference from the signalling power supply and, where necessary, the traction power supply. Early jointless track circuits were generally not traction immune. Several types of jointless track circuit are now available, many of which are immune to both a.c. and d.c. traction. The GEC "Reed" track circuit was also developed to be used as both jointless and conventionally jointed track circuits, immune to both a.c. and d.c. traction. However, problems were encountered with the operation of the track circuit in its jointless form in certain situations and it is at present only used on BR as a conventional jointed track, often in areas with both ac. and d.c. traction together. It will however be described  in this  section. Due to the absence of insulated joints, the limits of jointless track circuits cannot  be  as precisely defined as for conventional track circuits. This generally presents  no problem  on plain line but, as the extremities of track circuits through pointwork need to  be  precisely defined to ensure safe clearances, block joints must be used. Track circuits  which  define precise clearances will thereftill need insulated joints  although  the  same  jointless  track circuits transmitters and receivers may often be used. Joints are also  required  at  the  extremities of jointless sections where they adjoin conventional track circuits. To permit the filters to operate correctly, each type of track circuit will often have  a minimum length. This may be of the order of 50 metres. Shorter track circuits may  therefore be unsuitable for use with jointless track circuit equipment. To permit adequate flexibility in the positioning of track circuits to meet all requirements, at least four distinct frequencies must be used, two on each line of a double line railway. This prevents interference between track circuits on adjacent lines. An early type of jointless track circuit, the Aster 1 watt, employed six frequencies, three for each line. On railways with more than two parallel lines, each set of frequencies could be used on alternate lines. Aster track circuits were not traction immune. A further development was the Aster U type. This employed only four track frequencies (two on each line) and was also not traction immune. To permit alterations (addition or removal of a track circuit) with only two frequencies per line, the track circuit may be centre fed if required. It is still a simple example of a jointless track circuit to study and  will be described first. 8.1 Aster Track Circuits The connection of the transmitter and the receiver to the rails is by secondary coils of the  track transformer. This defines the approximate position of the boundary between the two track circuits. The tuning units operate in conjunction with the inductance of the rails to form filters. In the diagram below, L1-4 are the inductances of the rails between the tuning units and the track transformer. At frequency F1, C2 and L6 are series  resonant  and  are therefore  an  effective  short  circuit to F1. The remaining components present  a parallel  resonant circuit of high  impedance to the F1 signal. F1 will thus reduce in amplitude progressing from Tuning Unit  1  to Tuning Unit 2. A similar situation arises for F2 with L5 and C1 acting as a series resonant short circuit. Four frequencies are used. 1.7KHz and 2.3KHz would alternate on one line and 2.0KHz and 2.6KHz would alternate on the other line. Track circuit length may be from 50m to 1000m long. By using a centre feeding Transmitter the track circuit may be extended up to 2000m either as two individual track circuits or a single long track circuit (the TPR circuit would of course include both receivers in this case). Aster U Type Track Circuit - Tuned Area Between Adjoining Tracks 8.2 ML Style T1-21  Track  Circuits The TI21 track circuit is similar in operation to the Aster track circuit already described, but has been designed to provide traction immunity. This track circuit requires a tuned length of 20m with the tuning unit F1. acting as a short circuit to F2 and Tuning Unit F2 short circuiting F1. Transmitter and Receiver connections are made via the respective tuning units. The security of the system is enhanced and traction  immunity provided by operating  each track circuit at two frequencies 34 Hz apart and modulating between them at a rate of 4.8 Hz. The receiver must detect both frequencies and the correct rate of modulation to energise the follower relay. It is considered that any signal produced by the electric traction supply  and its harmonics would be unable to simulate the track circuit signal for long enough  to energise the relay. For traction return purposes, parallel tracks are normally bonded together at regular  intervals. It is therefore not possible to re-use the same frequencies on third and fourth parallel tracks. Eight frequencies are available, four of  which  are preferred  and thus used on two track lines. The remaining four are used on third and fourth lines when required. A minimum length of 200m is required (300m if centre fed) and a maximum of 1100m is permitted (1000m per end if centre fed). Length can be reduced to 50m in a special low power mode of operation but the adjoining track circuits either side must then be connected with the transmitter nearest to the low power track to prevent the signal feeding through and falsely energising the receiver of the next track of the same frequency. On d.c. electrified lines, double rail traction return is usually required and impedance bonds will be required at feeder points and wherever adjacent tracks are bonded together. They  will not be required at the extremities of  the  tracks. Special resonating units are provided to resonate the bonds to the track circuit frequencies. Tl21 Transmitter & Receiver Connections 8.3 GEC Type RT Reed Track Circuits The GEC Type RT Jointless Track Circuit operates by detecting the current in the track circuit loop, as opposed to the voltage at the end of the track circuit like the jointless track circuits already described. It is always centre fed in the jointless mode, although the last track before block joints may be end fed. The Type RT Track circuit  may also be used  as a jointed track circuit, in which case it may be centre or end fed. Each track circuit operates at a frequency defined by a mechanical reed filter in the transmitter. The filter is very stable in frequency and has a bandwidth of less than 1 Hz. This allows track frequencies to be only 3 Hz apart, using 366, 369, 372, 375, 378, 381 and 408 Hz. On each line a four frequency cycle should be maintained. The transmitter is connected directly to the rails. The receiver only responds to its own frequency due also to a reed filter at its input. When used in its jointless configuration, each track circuit is centre fed. The receiver is energised by an aerial positioned between the rails. The aerial comprises 36 cores of a 37 core cable wired to give a 36 turn inductive loop. The 37th core is connected to a series resonant shunt which terminates the track circuit. At any frequency other than the track circuit frequency, insufficient current will flow in this loop to energise the receiver. As the limits of the track circuit may not extend as far as the rail connection to the loop, the ends of adjacent tracks must overlap as shown. The inefficiency of the shunt has led to problems in practice on B.R. where track circuit lengths are often short and/or more than two parallel tracks occur on electrified lines. The result is that signals from nearby track circuits of the  same  frequency  feed  through sufficiently  to be detected  by other receivers. This type of  track circuit  is only used on B.R.  in its conventional jointed form, where it provides dual (a.c. & d.c.) traction  immunity. Receiver connection is then directly to the rails. Tracks may be either centre or end fed. GEC Reed Track Circuit Arrangement for Jointless Operation 8.4 Westinghouse FS2500 Track Circuit This is another recent addition to the range of jointless track circuits available. The location equipment can be mounted on a standard BR930 relay rack. The track circuit is traction immune and its circuitry includes the use of microprocessors to process the track circuit signal digitally. It employs a similar signal to the TI21 track circuit, a modulated ac. signal where the level of modulation and the modulation frequency must be correct to indicate a clear track circuit. An interface has been designed to connect direct to a Solid State Interlocking (SSI) trackside module without the use of a track relay. The receiver has a built in "slow to operate" feature. It employs four sets of frequencies, two on each line. It may be centre or end fed. In addition, the receiver may be positioned remote from the end of the track circuit, useful in tunnel situations. Different termination arrangements are used in each situation. For a series of end fed track circuits, each transmitter and receiver is connected to the rails via a tuning unit. The tuning unit performs three functions:- A high impedance to its own frequency A low impedance to the frequency of the adjacent track circuit. This prevents transmission of the signal through to the next track circuit of the same frequency and shunting of the track by a train outside its nominal limits A transformer for transmission of the signal from transmitter to rails and from rails to receiver Adjustment of the signal level can be performed at both transmitter and receiver. It is usual for the transmitter to be set up on installation according to the length of the track circuit. Finer adjustment according to individual track characteristics is at the receiver. At the end of a jointless section, an end tuning unit, which is slightly different to the normal tuning units for track circuit separation, is connected across the rails. The same tuning unit is used at any centre feed to provide a high impedance connection between transmitter and rails. It can also be used in conjunction with an impedance bond where connection between tracks, to traction feeders or to conventional jointed track circuits. Air cored inductors (often referred to as SI units) are used where it is necessary to make earth connections to the track without direct connection to the rails. These are installed in the centre of the tuned area between tuning units. Intermediate receivers can be installed within a track circuit to indicate the occupation of part of a track circuit, for example approaching level crossings. Maximum track circuit length is 1.3km end fed (although SRA adopts a maximum of 900m). Minimum track circuit length is 50 metres. 8.5 CSEE UM71 Track Circuit The operation and configuration is similar to other types of traction immune jointless track circuits. It is not intended to repeat the details contained in earlier sections. Only the differences will be identified. Instead of the tuning unit, separate tuning and matching units are provided at the trackside for connection and track circuit separation. Resonating units for each frequency are available for use with impedance bonds. Maximum length is 600m and minimum length is 50m. Length may be extended up to 2km  by the use of compensating capacitors connected at intervals between the rails. Two types of equipment are in use, the "T1" for most normal applications and the "T2" for extended connections in tunnel situations. 9. IMPULSING TRACK CIRCUITS Rusty rails, as may occur on lightly used lines, will tend to cause wrong side failures whereby vehicles will remain undetected. To overcome this problem two basic methods are used. One is to weld a stainless steel strip onto the surface of such rails in critical areas; the strip will not go rusty. The other is to use impulsing track circuits. The principle of an impulse track circuit is to  use  high  voltage  "spikes"  to  be fed  to the rails. If a train or single vehicle occupies the track the voltage of these spikes is enough  to break down the resistance caused by the rust on the rails. The voltage spikes applied to the track have insufficient energy to electrocute personnel on the line, although their voltage is  of the region of 150V. Frequency is of the order of 3 spikes per second. Although a higher a.c. voltage on the rails would produce a similar effect, the power consumption would be excessive. An impulse type track circuit generally consumes no more power than a conventional track circuit. Two types, the Jeumont and the Lucas track  circuit, are in use on B.R. although they are not very common. SRA uses the Jeumont track circuit extensively. On the Jeumont track circuit, / a typical voltage waveform on the rails is shown on the following page. The receiver detects the correct polarity of the pulse (important in the event of a failed insulated joint) and the correct ratio of voltage and duration between the positive and negative parts of the waveform. It must also provide an output to the track relay in the interval between pulses. The arrangement of connections to the track is also shown. Jeumont Track Circuit - Typical Rail Voltage Waveform Jeumont Track Circuit - General Arrangement of Rail Connections It is not intended to describe the construction and operation of Jeumont tracks in detail. They are generally considered to be traction immune due to the low frequency of operation (3Hz). Due to the high voltage on the rails, they can be used for long track circuits, up to 1500 metres on electrified lines and 3km on non-electrified lines. Double rail track circuits on d.c. electrified lines will be connected at each end via an impedance bond. The relay is peculiar to the Jeumont track circuit and is designed to operate with the receiver. 10. RAIL CIRCUITS AND OVERLAY TRACK CIRCUITS Unlike a track circuit, which detects the absence of a  train, a rail circuit detects  the presence of a train. It may be used at automatic level crossings, for approach control of  signals, or in place of treadles on high speed lines. It has also been used in situations such as bump marshalling yards to control the operation of points, due to its higher speed of operation when compared with the track circuit. To detect the absence of trains, a rail circuit is not fail safe. The simplest form of rail circuit consists of a standard track relay connected in series with its feed across the rails. A train or vehicle on the track will then complete the circuit, energising the relay. An alternative to the rail circuit is the overlay track circuit. It uses a much higher frequency signal than conventional track circuits. The inherent impedance of the rail limits its extent of operation, so insulated joints are not required and it may be overlaid on most conventional track circuits. 11. SHUNT ASSISTERS As mentioned earlier, increasing problems are being experienced with track circuits due to the introduction of lightweight vehicles with improved riding characteristics. Obviously, unreliability of track circuits puts the integrity of the whole signalling system at risk. Large scale replacement of existing track circuits with those that will operate satisfactorily with the new vehicles may not be practical or economic. If it can be shown that the problem is confined to certain types of train, it may be possible to deal with it by modifying the vehicles instead of the track circuits. On B.R. certain classes of one and two car diesel multiple unit vehicles are being fitted with shunt assisters. These operate by applying a high voltage (compared to most track circuits) signal to the rails. This is outside the frequency range of normal track circuit operation (50khz) and does not affect the operation of relays and/or receivers. It does however break down the resistance between rail and wheel and allow the track circuits to operate more reliably. Such a solution is not without problems. The on-board equipment must be  monitored  for correct operation and its failure could prevent further operation of the  vehicle  until  the problem has been rectified. It is not therefore a substitute for ensuring reliable track circuit operation for normal traffic. 12. ALTERNATIVES TO TRACK CIRCUITS In locations where track circuits cannot be installed or where a long track circuit is required, axle counters may be a possible solution. A set of axle counter equipment is generally more expensive than a single track circuit. If one set of axle counters can substitute for several track circuits, the overall cost may be cheaper. If required, axle counter sections can be designed to overlap, reducing the overall quantity of equipment. The track equipment may also be located remotely from the electronic logic equipment. Axle counters will be covered in more detail in a later section. Although not yet used in any working system, the use of Automatic Vehicle Identification transponders on vehicles has the potential for use as a train detection system. An esential requirement is, of course that all vehicles are fitted with transponders and that they can be proved to be functional.

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Deepu Dharmarajan -
Posted 91 days Ago

TRACK CIRCUIT BONDING

CONTENTS 1.Introduction 2. Fouling & Clearance Points 3. Positioning of Insulated Joints 4. Jointless Track Circuits 5. Bonding of Rails 6. Track Circuit Interrupters 7. Other Information on Bonding/Insulation Plan 8. Notes for Practical Work 1. Introduction   A modern signalling installation will use many track circuits. The limits of each track circuit must be precisely defined and the track circuit must be connected to operate safely and reliably, even through the most complex points and crossings. In many cases this will involve the installation of insulated joints, although jointless track circuits are available to suit many applications. An insulated joint is relatively simple to provide in jointed track. The fishplate and its bolts are insulated from the rail and an "end­ post", a piece of insulating material of similar shape to the rail cross section, is inserted between the rail ends. In welded rail, this operation is much more complex. Either the rail must be cut and an insulated joint inserted or a length of rail is removed and a pre-assembled joint in a section of rail is substituted. The rail is then welded. Both operations involve the adjustment of the rails to allow for internal stresses. After installation an insulated joint will generally be weaker than the rail on either side. Therefore, although insulated joints are essential, their position must be chosen correctly and the number of joints minimised. In addition, it may be desirable to avoid joints in positions of greatest wear, vibration or stress on the rails. The positioning of insulated joints may often be a compromise between the requirements for an ideal track circuit and the practicalities of permanent way construction and maintenance. For correct operation, the two rails of each track circuit must both be electrically continuous between all extremities of the track circuit. The rails must also be insulated from each other. This requirement is just as important through points and crossings as on plain line. There should be no position within a track circuit where a vehicle can be totally undetected. Each track circuit must operate reliably and must as far as is practical fail safe. Any normal failure mode should result in the track indicating occupation by a train. In areas where electric traction is employed, one or both rails will be used for the return traction current. The track circuit arrangement must permit an adequate traction current return path while maintaining safe operation of track circuits. In addition to the signalling plan it is customary to prepare a plan showing the arrangement of the track circuits. Instead of a single line for each track, it will show each rail individually. Its main purpose is to show the bonding and insulation arrangements for all track circuits. In addition, it may show other useful information such as position of cable routes and locations, overhead structures, traction power supply connections, earth bonding and further details of the track circuit equipment. 2. Fouling & Clearance Points Many insulated joints are positioned to prove clearance in the vicinity of points and crossings. The following terms will be used in these notes to describe the positioning of insulated joints. The fouling point is the position at which the extremity of a vehicle on one track is clear  (by an adequate margin) of a movement on a converging line. The placing of an insulated joint at this position will not, however, ensure sufficient clearance. The clearance point is the position at which an insulated joint must be placed to ensure a vehicle stands beyond the fouling point. The distance of this from the fouling point will be determined by the longest overhang of all vehicles operating on the line. It follows that a joint which is intended to prove clearance must be positioned beyond the clearance point. although the signalling plan is not of adequate scale to show these joints accurately the track circuit bonding plan, or insulation plan, must be of a large enough scale to do so. If necessary, critical measurements must be taken from permanent way construction drawings. The limits of jointless track circuits cannot be precisely defined and cannot accurately determine clearance points. Even if jointless track circuit equipment is used through points and crossings, insulated joints will be needed to define clearance points and to electrically separate opposite running rails. Determination of Fouling and Clearance Points 3. Positioning of Insulated Joints The positioning of insulated joints must fulfil all of the following requirements:- Within any track circuit, the two rails must always be of opposite polarity. Unless adjacent track circuit signals employ different frequencies, the polarity (d.c. tracks) or phase (a.c. tracks) must be opposite on each side of all insulated joints. This is equally important whether the joint separates two track circuits or the two rails of the same track circuit Where it is unavoidable to stagger block joints (i.e. they are not exactly opposite), the separation must be limited so that complete vehicles cannot remain undetected Separation of a staggered pair of joints from an adjacent pair of joints (whether staggered or not) must not result in a vehicle being undetected. Critical clearance points cannot be defined by staggered  joints. Minimum track circuit length must be greater than maximum vehicle wheelbase. Maximum and minimum track circuit lengths must be within the specified range of operation of the type of track circuit used. Most railways now employ a high degree of standardisation of permanent way components. This will often restrict the position of insulated joints within points and crossings. Preferred positions must be used wherever possible to avoid additional cutting of rails and subsequent track maintenance. The use of these preferred positions will often result in joints being staggered. At a turnout, if there is a choice between joints in a high speed or low speed line, the low speed line is usually preferred. In cases where electric traction employs a single rail return, joints in plain line will usually be provided in one rail only. Pairs of joints will occur however in points and crossings and where it is desired to change the traction return rail from one side to the other. The above rules for staggering will still apply. In areas with double rail traction return, jointless track circuits must be used or joints must be provided in both rails, together with impedance bonds for continuity of traction current past the joint. 4. Jointless Track Circuits For adjacent jointless track circuits of the same type, no joints are necessary unless clearance points must be accurately defined. For replacement of signals and defining the end of an overlap, tolerances of 5-10 metres are usually acceptable. Adjacent jointless track circuits of the same type must be of different frequencies. Where a jointless track circuit adjoins a jointless track circuit of a different type, block joints will usually be needed due to different operating characteristics. A filter designed to operate with one type of track circuit is unlikely to discriminate correctly  between signals of a different type of track circuit. Where any extremity of a jointless track circuit must be accurately positioned for clearance purposes, insulated joints must be provided. 5. Bonding of Rails Each rail must be bonded to give electrical continuity throughout the track circuit. The arrangement of bonding may be dependent on traction power supply arrangements. For fail safe operation feed/transmitter connections must be at one extremity of the track circuit and relay/receiver at the other. Between the feed and relay connections, the bonding must as far as possible be fail safe. If the rails are not welded together or otherwise bonded (e.g. for traction current) the signal engineer must provide adequate bonding throughout the length of the track circuit. The safest way of bonding the rails together is in series. On plain line this is the only practical method of bonding so no problems will arise. Within points and crossings, all track circuits will have additional branches which must also be bonded. All sections of a track circuit should still be bonded in series but this may not be possible in all cases due to traction requirements. A certain amount of parallel bonding may be necessary. The two diagrams on the next page demonstrate the difference between series and parallel bonding. In the first example, the rails in the turnout are parallel bonded. A break in any bond or rail in this section of track could leave a vehicle undetected - the track circuit will indicate clear when a train is standing on part of it. If the connections are rearranged as in the second example, only very short sections of rail are now parallel bonded. Other than in these short sections, a break in a bond or a rail will cause the relay to drop, indicating an occupied track. Parallel Bonded Track Circuit Through Turnout Series Bonding of the Same Track Circuit Even in non electrified areas, there will always be short branches of a track circuit which cannot be connected in series (e.g. where the switch and stock rail adjoin). When the arrangement of track circuit bonding and insulation is being designed, the length of these branches should be kept as short as possible. 5.1 Non-electrified Areas It is usual, although not essential , to install insulated joints in both rails where electric traction is not used. In this case, series bonding should be employed throughout. To maximise the amount of series bonding, certain portions of rail through points and crossings may be common to two adjacent track circuits. In general, this should not cause a problem but in complex track layouts, the bonding and insulation must be checked very carefully to ensure that the use of a common rail between several nearby pairs of track circuits does not provide an electrical path for false operation of a track circuit and does not cause any track circuit to be shunted by trains outside its limits. Track Circuits Sharing a Common Rail 5.2 Single Rail Traction Return On many electrified lines, particularly those with a high voltage a.c. supply; traction return is via one rail only. The signal engineer still has exclusive use of the other rail. The two rails are normally designated the signalling rail and the traction rail. Of course, the signalling equipment must always use both rails. Track circuits connected in this way are described as single rail track circuits. The signalling rail will be series bonded. The traction rail must be connected to give the lowest impedance path back to the feeder station. Track circuits should be connected so that as much as possible of the traction rail bonding is in series. Often, however, parallel bonding must be accepted in the traction rail. A typical example of bonding for a single rail track circuit is shown below. The signalling rail is connected to provide the maximum amount of series bonding. The traction rail shows a significant amount of parallel bonding. Typical Bonding - Single Rail Track Circuit 5.3 Double Rail Traction Return On some lines, particularly those with d.c. traction, a lower supply voltage increases  the traction current. This will often require both rails to be used for traction return.  Track circuits must therefore operate safely and reliably while sharing both rails with the much larger traction currents. To allow traction currents to pass conventional insulated joints, impedance bonds are used. Where two double rail tracks adjoin, the centre connections of the two impedance bonds are joined together. The ends of each coil are connected to the rails on either side. Where a double rail track circuit is of a jointless type, impedance bonds are not normally needed where tracks of the same type adjoin. At the end of a section containing  a number of jointless track circuits, insulated joints are usually required and an impedance bond (resonated to the track circuit frequency) will be needed to pass the current around the insulated joint. Impedance bonds are also needed wherever the traction return must  be  connected  to  the supply at a feeder point and where adjacent roads are bonded  together  to  reduce  the impedance of the return path. Plain line track circuits are inherently  series  bonded.  Through  points  and  crossings, however, if the double rail traction return is to be  continuous, a proportion of parallel bonding is required. Although it prevents proof of continuity of  the  track  circuit  and  its  bonding,  parallel bonding must be accepted as the only means of providing a low impedance traction return. Additional security is usually given by duplicating some or all of the  rail to rail bonds (which of course will all be traction bonds and must consist of a suitably sized conductor).  Even in series bonded sections of rail, bonds or jumpers are often duplicated for reliability reasons. It must be remembered that maintenance routines must include regular checking of the integrity of these bonds because the disconnection of a single bond will not become evident as a track failure. Double Rail Track Circuit with Parallel Bonding 5.4 Transition Between Single & Double At the end of double rail sections, where they adjoin single rail track circuits. Insulated joints will be required in both rails. An impedance bond will generally be needed for the double rail track. Its centre connection in this case will be bonded to the traction rail of the single rail track circuit. Typical Connections Between Single and Double Rail Track Circuits Where double rail tracks are in general use, even a small section of single rail return will increase the impedance of the traction return path. Single rail tracks in this situation are kept as short as possible. In some cases, two separate track circuits may be provided  where  a long track includes both points and plain line, even if one single rail track circuit  could perform adequately over the total distance. 6. Track Circuit Interrupters Track circuit interrupters are used to detect that a train has been derailed by a set of catch (trap) points by maintaining the track in the occupied state. The reason for this is that a derailed vehicle may be completely clear of the rails while still in a position which would be foul of other movements. If track circuit interrupters are provided, the following rules will generally apply to the track circuit bonding:- The track circuit interrupter will be insulated from the rail upon which it is mounted It will be bonded  in series with the opposite rail to the one upon which it is mounted Traction current should not pass through the track circuit interrupter. If mounted on a double rail track circuit, the interrupter must be connected in a separate circuit. A contact of the interrupter repeat relay must be included to cut the TPR circuit of its associated track 7. Other Information on Bonding/Insulation Plan   The bonding or insulation plan will often show other additional information such as:- Position of overhead electrification structures Bonding between overhead structures and traction return rails. Positions of locations, cable routes and signal structures. 8. Notes for Practical Work This section includes some practical work to assist understanding. You will be supplied with a number of blank track layouts, each accompanied by an extract from the signalling plan to show the track circuiting arrangement required. In the vicinity of points and crossings, the position of all joints will be restricted by the standard track components available. These are shown by a light line drawn at right angles to the rail. If a joint is required at any of these positions a bold line should be drawn. On plain line, you may assume that joints can be provided at any suitable position. For each plan, you should firstly show the bonding arrangement required for a non­ electrified line. You should also repeat at least one example for an a.c. electrified line which has single rail traction return throughout and for a d.c. electrified line using double rail traction return wherever possible. Feed/transmitter and relay/receiver connections must be shown for each track circuit. Bonds between rails and all insulated joints should be shown. Bonding necesary for track circuit continuity must also be added. The two rails should be distinguished by the use of  colour or by drawing one rail bold. Remember the following requirements for correct operation of track circuits on non­ electrified lines:- Insulated joints are normally placed in both rails. Except where distinct frquencies are used, the polarity or phase of the rails on opposite sides of an insulated joint must be opposite. Bonding must as far as possible be in series. Jumper bonds must be kept as short as practical. Track circuit minimum and maximum lengths must be observed. Staggered joints (and the amount of stagger) must be minimised and be within safe limits. Staggered joints must not be used to prove clearance unless at a safe distance. The plan should therefore distinguish between the two rails to ensure that the polarity/phase change at each joint is provided. Coloured, bold or dotted lines are all acceptable methods.

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Deepu Dharmarajan -
Posted 98 days Ago

THE PURPOSE OF SIGNALLING

  CONTENTS 1.Introduction 2.The Problems to be Solved 3.Basic Requirements 4.Lineside Signals 5.The Absolute Block System 6.Interlocking of Points and Signals 7.Single Lines 8.Further Developments 1. INTRODUCTION In general, the railway traveller assumes  that  his  journey  will  be safe. This  high  standard of safety which is taken for granted is the result  of  a  long  history  of  development.  As human errors and deficiencies in safety systems become evident, often as a result  of  an accident, improvements are made which are then incorporated into new generations of equipment. This is certainly true of railway signalling. It also appears to  be  a continuing  process.  We have not yet reached the situation where absolute safety can be assured. It is useful to start by looking back at some of the early history of signalling development. In the early days of railways, trains were few and speeds were low. The risk of a serious collision between two trains was minimal. Better track and more  powerful  locomotives allowed trains to run faster (requiring greater stopping distances). Railway traffic increased, requiring more and larger trains. The risks thus became greater and some form of  control over train  movements  became  necessary.  The need  for railway signalling had been identified. 2. THE PROBLEMS TO BE SOLVED 2.1 Collision with a Preceding Train When one train follows another on to the same section  of  line,  there  is a  risk  that,  if  the first train travels more slowly or stops, the second train will run into the rear of the first. Initially, trains were  separated  using  a system  of  "time  interval"  working,  only  permitting a train to leave a station when a prescribed time had elapsed after  the  departure  of  the previous train. Although this reduced the risk  of  collisions, a minimum safe distance between trains could not be guaranteed. However, in the absence  of  any proper communication between stations, it was the best that could be achieved at that time. 2.2 Conflicting Movements at Junctions Where railway lines cross or converge, there is the risk of two trains arriving simultaneously and both attempting to enter the same portion of track. Some  method  of  regulating  the passage of trains over junctions was therefore needed. This should ensure that one train is stopped, if necessary, to give precedence to the other. 2.3 Ensuring that the Correct Route is Set Where facing points are provided to allow  a train  to take alternative  routes,  the points  must be held in the required position before the train is allowed  to  proceed  and  must  not  be moved until the train has· completely passed over the points. Depending on the method of point operation, it may also be necessary to set trailing (i.e. converging) points to avoid damage to them. 2.4 Control of Single Lines Where traffic in both directions  must  use  the same single  line,  trains  must  not  be allowed to enter the single line from both ends at the same time. Although this could in theory be controlled by working to a strict timetable,  problems  could  still  arise if  trains were delayed or cancelled. 3. BASIC REQUIREMENTS We therefore have the basic requirements of any railway signalling system. The method of implementation has changed over the years but the purpose remains the same:- To provide a means of communicating instructions to the driver (signals) to enable him to control his train safely according to the track and traffic conditions ahead. To maintain a safe distance between following trains on the same line so that a train cannot collide with a preceding train which has stopped or is running more slowly. To provide interlocking between points and the signals allowing trains to move over them so that conflicting movements are prevented and points are held in the required position until the train has passed over them. To prevent opposing train movements on single lines. All the above requirements place restrictions on train movements, but it is vital that the signalling system will allow trains to run at the  frequency  demanded  by  the  timetable  to meet commercial requirements. This must be done without reduction of safety below an acceptable level. Signalling involves not only the provision of  equipment  but the adoption  of  a consistent  set of operating rules and communication procedures which can be understood  and implemented by all staff responsible for railway operation. 4. LINESIDE SIGNALS It will probably be evident that the decisions regarding the movement of two or more trains over any portion of the railway can only be made by a person on the ground  who  has  sufficient knowledge of the current traffic situation. His decision must be passed on  to the driver of each train passing through his area of control. In the early days railways employed policemen whose duties would include the display  of  hand signals to approaching trains. As the policemen also had many other duties, it soon became impractical for them to be correctly positioned at all times. Fixed signals of various designs, often boards of different shapes and colours, were provided. The policeman could then set these and attend to his other duties. The simplest signals would only tell a driver whether or not he could proceed. From this evolved a standard layout of signals at most small stations; a "home" signal  on  the approach side controlling entry to the station and a "starting" signal  protecting  the section  of  line  to the next station. Between these signals, each train would be under the direct control of the policeman. These signals could give only two indications,  STOP  or  PROCEED.  They therefore became collectively known as "stop" signals. As line speeds increased, "distant" signals were  introduced  which gave advance warning  of the state of stop signals ahead. A distant signal could be associated with one or more stop signals and would be positioned to give an adequate braking distance to the first stop signal. It could give a CAUTION indication to indicate the need to stop further ahead or a CLEAR indication, assuring the driver that the stop signal(s) ahead were showing a proceed indication. With the addition of distant signals, trains were no longer restricted to a speed at which they could stop within signal sighting distance. It is important to understand the difference between stop and distant signals. A train  must never pass a stop signal at danger. A distant signal at caution can be passed  but  the driver  must control his train ready to stop, if necessary, at a stop signal ahead. The earliest signals were "semaphore" signals (i.e. moveable boards). To enable operation at night, these often had oil lamps added. With the advent of reliable electric lamps, the semaphore signal became unnecessary and a light signal could be used by day and night. Red is universally used as the colour for danger while green is the normal colour for proceed or clear. Initially, red was also used for the caution indication of distant signals but many railway administrations changed this to yellow so that there was no doubt that a red light always meant stop. If necessary, stop and distant signals can be positioned at the same point along the track. Alternatively, certain types of signal can display three or more indi tions to act as both  stop and distant signals. 5. THE ABSOLUTE BLOCK SYSTEM Although time-interval working may seem crude, it is important to remember that nothing better was possible until some means of communication was invented. The development of the electric telegraph made the Block System possible. On many railways, time-interval working on double track lines is still the last resort if all communication between signal boxes is lost. 5.1  Block Sections In the Block Signalling system, the line is divided into sections, called "Block Sections". The Block Section commences at the starting signal (the last stop signal) of one signal box, and ends at the outermost home signal (the first stop signal) of the next box. With Absolute Block working, only one train is allowed in the Block Section at a time. The signalman may control movements within "Station Limits" without reference to adjacent signal boxes. The· accompanying  diagram shows a block section between two signal boxes on a double track railway. To understand the method of working, we will look at the progress of a train on the up line. Signalbox A controls entry to the block section but  it is only  signalman  B who  can  see a  train leaving the section, whether it is complete (usually checked by observation of the tail lamp) and who thus knows whether or not the section is clear. Signalbox B must therefore control the working of the UP line block section. Similarly, signalbox A controls the DOWN line block section. 5.2 Block Bell The signalmen at each end of a block section must be able to communicate with each other. Although a telephone circuit is a practical means of doing this, a bell is normally used to transmit coded messages. It consists of a push switch ("tapper") at one box, operating a single-stroke bell at the adjacent box (normally over the same pair of wires). The use of a bell enforces the use of a standard set of  codes  for  the  various  messages required to signal a train through the section and imposes a much greater discipline than a telephone, although a telephone may be  provided as well, often  using  the same circuit  as  the block bell. 5.3 Block Indicator This provides the signalman at the entrance to  the  section  with  a  continuous  visual indication of the state of the section, to reinforce the bell codes. It is operated  by  the  signalman at the exit of the block section. Early block instruments were "two position" displaying only two indications;  line clear and line blocked. Later instruments display at least 3 indications. The most usual are:- Line clear Giving permission to the rear signalman to admit a train to the section. Normal or Line Blocked Refusing permission, The signalman at the entrance to the section must maintain his starting signal at danger. Train on Line There is a Train in the block section. 5.4 Method of Working When signalbox A has an UP train approaching to send to box B,  the signalman  at  A will offer it forward to box B, using the appropriate bell code (so that signalman  B knows  what type of train it is). If the signalman B is unable to accept  the train for any  reason,  he will ignore A's bell, and leave the UP line block indicator at "Normal". If he is able to accept the train, signalman B will repeat the bell code back to box A, and  change the indication to "Line Clear". When signalman A sees  his  block  repeater go to "Line Clear", then he can clear his starting signals to admit the train to the section. When the train actually enters the section, signalman A sends the "Train Entering Section" bell code to box B. Signalman B will acknowledge  this by repeating  the bell code  back  to A, and turning the block indicator to "Train on Line". When the train leaves the block section at B, the signalman  there checks  that  it is complete by watching for its tail lamp. He then turns his block indicator to "Normal" again.  He also sends the "Train out of Section" bell code to A, which A acknowledges by repeating it back. The system is now back to normal, ready for the next train. On multiple track railways, a pair of block instruments as above is required for each line. 5.5 Extra Safeguards The basic three-position block system, as described, relies on the correct sequence  of operations for safety. A signalman could forget that he has a train in section and turn the indicator to "line clear", allowing a second train in. A detailed record (the train register) is kept of the actual times of train arrival and departure,  and the times at which the bell signals are exchanged. In most  places, additional safeguards have been added  to the basic system. An  electric lock on the starting signal will prevent it being operated unless the block  indicator is at line clear. Track circuit occupation  may  be used  to set the block  instruments  to ''Train on Line" if the signalman forgets to do so. Electric locking may also be used to ensure that signals are operated for one train movement only and replaced to danger before another movement is permitted to approach. Although it is unusual for absolute block working to be installed on any new signalling installation today, there are many railways on which it is in widespread use. The  block system, by ensuring that only  one train may occupy a section of line at any time, maintains a safe distance between following trains. 6. INTERLOCKING OF POINTS AND SIGNALS On all early railways, points were moved by hand levers alongside the points. They could therefore be moved independently of the signals controlling the movement of trains. A great improvement in safety (as well as efficiency) was possible by connecting the point switches via rodding to a single central control point (the signal box). Similarly the signals could also be operated by wire from levers in the signal box. With the control of points and signals all in one place the levers  could  be directly interlocked with each other. This had the following benefits:- Signals controlling conflicting routes could  not  be operated at the same time. A signal could only be operated if all  the  points  were  in  the correct  position. The points could not be moved while a signal reading over them was cleared. In early signalling installations, all point and signal  operation,  together  with  any interlocking, was mechanical. Although it was a great technological advance to be able to control a station from one place,  the effort required to operate the levers restricted  control of points to within about 300 metres from the signal box and  signals up to about 1500 metres. At large stations, more than one signal box would often be necessary. The possibility still existed for a signalman to set the points, clear the signal, the train to proceed and then for the signalman to replace the signal to normal. This could free the locking on the points before the train had completely passed over them. Signalmen's instructions usually required the complete train to pass over the points before the signal was replaced to danger. 7. SINGLE LINES On most single line railways trains are infrequent. It is not normally necessary for two trains to follow  each other closely in the same direction. Single lines were therefore  treated  in  the same way as a normal block section with the important  extra condition  that  trains could  not be signalled in both directions at the same time. To enforce this condition and also to reassure the driver that he could safely enter the single line, some form of physical token was used as authority to travel over the single line. On the simplest of systems only one token existed. This caused problems whenever the pattern of service differed from alternate trains in each direction. If the timetable required two trains to travel over the single line in the same direction, the driver of  the first train would  be shown  the token (or train staff  as  it is commonly  known) to assure the driver that no other train was on the single line. His authority to enter  the single line would however be a written ticket . The following train would convey the train staff. Although workable, this system  would  cause  problems  if trains did not work strictly to the timetable. A further improvement was to provide several tokens, but  to  hold  them  locked  in instruments at either end of the single line. The instruments would be electrically interlocked with each other to prevent more than one token being withdrawn at a time. The one token could however be withdrawn from either instrument. If the single line block equipment fails, many railways employ a member of the operating personnel as a human token. The "pilotman", as he is usually known, will either travel with the train or instruct the driver to pass through the section. No other person may allow a train on to the single line. Operationally, this is the equivalent of the train staff and ticket system described earlier. 8. FURTHER DEVELOPMENTS The main functions of the signalling system  had  now  been defined,  although  they  were  to be continuously improved as the available technology developed. All  signalling  systems would be required to maintain a safe distance between  trains, interlock points and signals and thus prevent conflicting movements, and provide the necessary information so that the speed of all trains can be safely controlled. In recent years, the signal engineer has been asked to provide further facilities within the general scope of the signalling system. These include, train information to the operating staff, train information for passengers, detection of defective vehicles, identification of vehicles and the increasing automation of tasks previously carried out by humans. The technology exists to completely operate a railway without human intervention although the level of automation desirable for a particular railway is for that railway administration to decide. Factors such as cost, maintainability, reliability, staffing policy, passenger security and sometimes political considerations must be taken into account. In many cases the final decision on the type of  signalling  to  be  provided  is  outside  the direct control of the signal engineer. However, he should always  endeavour  to provide  the best possible information and propose cost-effective solutions  to particular  problems so that the best decisions can be made.

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Posted 120 days Ago

The digitalisation of railways drives safer trains

With the expected introduction of 5G on railways, which will inevitably result in the application of more technologically advanced capabilities – such as IoT or AI – Huawei is on hand to support railway operators in improving their digitalization efforts, enabling advances in safety and security across all aspects of the railway, including construction, operations, and maintenance. sk a railway operator what the number one reason for the digitalisation of operations is, and the reply will be, ‘to ensure safety’.  Digitalisation  efforts in the railway sector have been focused on this most crucial aspect, followed closely by efficient management of resources and operations and improving the user experience. Ask a railway operator what the number one reason for the digitalisation of operations is, and the reply will be, ‘to ensure safety’. Robust connectivity for uninterrupted services is absolutely essential for the signalling and telecommunication setup, or the ‘heart’ of the railway system. This is what controls the performance of the network, ensuring that there are no intra-train collisions or derailments. It is no secret that digitalisation has improved this space by leaps and bounds today. And, with the expected introduction of  5G  in the control command railway system, trains will have access to hi-tech capabilities – such as Internet of Things ( IoT ) and artificial intelligence ( AI ) applications – that will be able to transmit more data from trackside to control centres, enabling urban rail to manage critical infrastructure even more efficiently and increase safety for the overall sector. Connectivity also helps passengers to feel more comfortable and safer on their train journeys. This feeling of safety is more from a psychological perspective of being in control should an emergency arise, knowing that they can get timely information updates, contact family members and so on. End-to-end smart safety management Safety and security are central elements for any train operation and are required through all stages, from development and construction to operations and the maintenance of the railway line. Smart construction Safety and security start at the initial stage of rail construction itself. AI technology is applied for the identification of risks and improvement of safety standards, real-time monitoring and analysis and to also shorten the response time to any situation.  Huawei  has already participated in several rail construction sites for better safety management, adherence to safety practices and identification of safety violations, not to mention the quick response to any accidents. Additionally, since AI apps eliminate the need for human intervention, it is a cost-effective means of management. Smart Operations & Maintenance (O&M) Smart O&M is on the wish list of many railway operators to predict the health of their assets. IoT and AI apps are particularly useful for predictive maintenance, alerting railway staff of failures before they occur rather than the traditional way of reactive asset management. Huawei’s IoT solution for railways – eLTE-IoT and AR IoT Gateway – connects to different sensors and components along the railway track. Data can be sensed, captured and analysed such that, in the event of any abnormalities, the solution triggers an alarm to the train management system, allowing for preventive maintenance actions to be carried out effectively, with minimal human involvement.  

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Posted 100 days Ago

SWR achieves national standard in customer service

    South Western Railway (SWR) has been recognized for achieving the national standard in customer service, underpinning the company’s determination to improve the passenger experience in readiness for more people traveling on the railway after lockdown. Every aspect of SWR’s customer-facing departments underwent 18 months of rigorous examination before being awarded the ‘Putting the Customer First’ accreditation in January 2021 by Customer First UK.                                         SWR is now part of a select group of train companies to undertake assessment and receive this prestigious accreditation. Over 250 colleagues were surveyed or interviewed by Customer First assessors, who visited the train company’s operations facilities in Basingstoke and conducted numerous secret shopper visits at some of the busiest stations on its network, including London Waterloo, Clapham Junction, Basingstoke, and Southampton Central, to get a first-hand experience of being an SWR customer.  

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Deepu Dharmarajan -
Posted 93 days Ago

SRA SIGNALLING & ELECTRICAL SYMBOLS

  CONTENTS   The following pages show the signalling and schematic symbols used by the State Rail Authority of New South Wales on their plans and drawings. They are provided for reference use throughout the course. They are reproduced from SRA training material.   TRACK PLAN AND WORKING SKETCH SYMBOLS MECHANICAL SIGNALS POWER WORKED SIGNALS SINGLE LIGHT INDICATION WAYSIDE BUILDING AND STRUCTURES TRACK CIRCUIT DEVICES INTERLOCKING SYMBOLS APPARATUS HOUSINGS RELAYS AND CONTACTS CONTACTS OPERATED BY SIGNALS LOWER QUADRANT SEMAPHORE SIGNALS, INCLUDING BANNER SIGNALS CONTACTS OPERATED BY POINTS LEVER CONTACTS CATCH ROD CONTACTS MISCELLANEOUS APPARATUS 

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Deepu Dharmarajan -
Posted 85 days Ago

SOLID STATE INTERLOCKING

CONTENTS 1. Principles of Redundancy 2. General Description 3. Interlockings, Panel Processors and Panel Multiplex 4. Data Transmission 5. Long-line Link 6. Location Circuitry 7. Technicians Terminal 8. SSI Design & Testing 1. PRINCIPLES OF REDUNDANCY 1.1 General Solid State Interlocking (SSI) has been developed as an electronic replacement for present day signalling technology. The aim of the development is to provide a·safe and reliable system employing the latest micro-computer techniques and which will become the standard for the future. Solid State Interlocking is not just an interlocking system. The term SSI refers to a complete signalling system i.e. from the signalman's controls right down to the connecting cables to trackside equipment. It is therefore, not just  the electronic equivalent of a relay based interlocking as we know it. The use of SSI eliminates signalling relays (apart from track circuit and certain interface relays), and virtually all lineside multicore cables, which are replaced by twisted pair data links over which the signalling information is transmitted. Buildings to house the equipment are much smaller as a result. There is no need for remote interlockings. 1.2 Basic System Design Due to the highly predictable failure modes of relays as used in conventional  signalling circuitry the designers of signalling systems could produce circuits which  would remain safe under all probable fault  conditions. The failure modes of  electronic devices however are unpredictable, therefore, to ensure the safe operation of an electronic system a fresh approach to the problem is required. 1.2.1 Principles of Hardware Redundancy As previously stated the SSI system employs microprocessors to carry out the necessary functions. The basic principles of hardware redundancy involve the duplication of the microprocessors associated with a particular part of the system. Each processor working independently, which, under normal circumstances should provide the same results (outputs). In addition some means of comparing the outputs is required to ensure that they are in fact in agreement. In the event of a failure occuring in one or other of the microprocessors, the outputs would not coincide. The comparator would detect the disagreement, but (not knowing which of them was providing the correct output) would cause that part of the system to shut down. To prevent the highly improbable situation where the  second processor later fails  in the same way and so recreates agreement, an internal "security" fuse is blown, thereby disconnecting that part of the system permanently until replaced. This practice is adopted for that part of the system which connects directly to the external equipment (Trackside Functional Modules - to be dealt with later), where the effect of any fault would only cause localised interruption. Principles of Redundant Systems Practical Triplicated System - Majority Voting 1.2.2 Principles of Majority Voting If a duplicated system was adopted for the Central Interlocking electronics,. any disagreement detected by the comparator would shut down the entire interlocking leading  to a total interruption to the control of traffic. The method adopted in this case is to triplicate the interlocking microprocessors, again comparing the outputs. Should a failure occur in one of the interlockings its output would not coincide with the other two, which by their own agreement would out-vote the failed interlocking, causing it to disconnect itself from the system. Providing the two remaining interlockings continue in agreement, the system will still function, during which time relevant action can be taken to restore it to its normal operational state. Should however, one of the remaining interlockings develop a fault prior to the restoration of the triplicated system then the interlocking would shut down. Provided each interlocking is sufficiently reliable individually, the failure of a second interlocking before the initial fault has been rectified is very unlikely. This arrangement therefore not only ensures the safe operation of the interlocking but also greatly improves its availability. 1.2.3 Practical Approach to Majority Voting The arrangement for the Majority Voting system suggests the use of an intrinsically  safe voting circuit which implies the use of safety relays. This is clearly not an ideal  solution if we are trying to remove costly relays from the system. The system used  in practice is arranged so that all three interlockings compare their own outputs with the other two. In the event of a disagreement, the outputs can be "jammed", or disconnected and the module which does not agree with the other two can blow an internal "security"  fuse thereby disconnecting itself permanently from the system. Should the faulty module be unable to blow its own fuse the other two can jointly blow the fuse of the faulty module. The modules ability to blow these fuses is tested at regular intervals. Basic SSI System Structure 2. GENERAL DESCRIPTION 2.1 Introduction The basic structure of an SSI controlled installation will consist of:- An operating panel, which is outwardly identical to that of a conventional modern installation and which will contain panel multiplexing equipment to facilitate communications with the central interlocking, or A VDU based control interface such as the Integrated Electronic Control Centre (IECC). One or more microprocessor controlled Central Interlockings, the number being dependant on the size and complexity of the area to be controlled. A Technicians Terminal providing fault diagnostic, event logging and certain control facilities. A duplicated internal data link to provide communication between interlockings (where more than one is provided). A duplicated external data link between each interlocking and the  equipment  it controls at the trackside. A number of trackside modules which have access to the data links and include equipment which interfaces directly with the lineside signalling equipment, i.e. signals and points. 2.2 The Central Interlocking Each Central Interlocking comprises a number of modules housed in a standard  19" equipment rack. The different types of modules are briefly described below. 2.2.1 Multiprocessor Module (MPM) The controlling element of each Central Interlocking employs three modules of this type configured to carry out the interlocking  function, and are responsible for the safe execution of the interlocking process and issue of correct instructions (telegrams) to the lineside equipment. They also control the management of the redundancy system (majority  voting), of which they each form a part. The interlocking modules are triplicated for availability and the system will continue to operate as a duplicated system on failure of the first module. A multiprocessor module is also used as the diagnostic processor. In this case it is configured to monitor the flow of data on the data link and communicate with the technicians terminal. As it does not perform any vital safety functions and availability is not critical (i.e. it is acceptable simply to exchange the module if it should fail)  it  is not duplicated. Panel Processor, Multiprocessor and Memory Modules The following neon monitoring lamps are provided on each module which indicate when lit: Power module energised Fused supply redundancy management fuse intact System module on line 2.2.2 Panel Processor (P.P.M) The purpose of the panel processors, of which there are two in each system for availability, is to relieve the interlocking modules of the non vital task of servicing the signalmans panel. They communciate with the panel multiplexing equipment housed in the signalmans console and also peripheral equipment such as the train describer. Two neon lamps monitoring "power" and "system" are provided on each module. 2.2.3 Memory Module (MM) These plug into the front of MPM's and PPM's and carry the read only memory (EPROM) for the main processor. The EPROMS hold the fixed programs and the geographic data and thus configure the module to both function and site. The label colouring gives a quick guide to the type of memory module. Module Type Background Lettering Interlocking MPM Black Silver Diagnostic MPM Red Silver Panel Processor Module (PPM) Green Silver Simulator MPM Yellow Black Design Workstation MPM Silver Black Deswign Workstation PPM Silver Green RETB MPM Blue Silver RETB PPM Gold Black 2.2.4 Technicians Terminal The Technicians Terminal is provided as an essential aid to fault diagnosis in the SSI system. It incorporates a monochrome VDU, keyboard and printer and is connected to the central interlocking. The keyboard may be used to request specific status information or to apply restrictive controls to the interlocking. Information requested by the technician, fault messages, and restrictive controls appear on the VDU. The printer provides a hard copy. 2.2.5 Data Link Module (D.L.M.) These modules provide signal conditioning and matching between the data links (both internal and trackside) and the system electronic modules. As both data links are duplicated a DLM is required for each. They are found therefore at the Central Interlocking and also in the trackside apparatus cases. The pair of DLM's can link the two data links to six trackside modules. A neon lamp indicates the 110V power supply to the module. 2.3 The Panel Multiplex System When an SSI is used in conjunction with a signalman's panel it is necessary to provide an interface between the panel processors in the interlocking and the lamps and buttons on the panel. The interface equipment is referred to as the panel multiplexer but is not part of the standard SSI system. Consequently no one standard design exists and the equipment installed at any particular location will depend largely on who installed it and when. Toe two systems currently being used for the panel multiplexer are the Westinghouse S2 and the GEC RM. In simple terms the panel multiplex equipment forms one end of a TDM link  between the panel and the interlocking cubicle. The outputs from the  equipment  drive the panel indications directly and the inputs are the switch and push button contacts. A panel multiplex system is not required when the SSI is controlled from Integrated Electronic Control Centre (IECC) In this case, the panel processor communicates directly with the display system and other IECC sub-systems via a duplicated communications network. 2.4 The Data Link The data link is the means of communicating outputs from the interlocking to the external signalling equipment and the indications from that equipment back to the interlocking. It replaces the multicore cables used in a relay interlocking. The trackside data link consists of dedicated screened twisted pair cables (duplicated for availability) over which information between the interlocking and the  trackside equipment is transmitted in half duplex. As the link carries safety information and is the only connection between an  interlocking and the trackside  equipment that it controls, its security is important.  The  information in the form of input telegrams (to the interlocking) and output  telegrams  (from  the  interlocking), is protected by two levels of coding (Hamming and Manchester II coding)  and the use of high signal levels. A separate Internal Data Link will be required where more than one interlocking is provided and information has to be passed between interlocking cubicles, eg. route requests, aspect sequence or approach locking release. As this link is also used for vital information the same level of protection as the trackside data link is provided. 2.5 Trackside Equipment The modules at the trackside are located in conventional apparatus cases and are connected to the data link via Data link Modules identical to those at its Central Interlocking, and line termination units which provide surge protection. Each DLM has six outputs and can, connect up to 6 trackside modules. These modules known as Trackside Functional Modules (TFM) are divided into two types: Point Modules and Signal Modules. They convert  the data information received in output telegrams to power level outputs required by conventional lineside signalling equipment. The state of external signalling functions (e.g. detection, track circuits, filament  proving) is detected by these modules and returned to the Central Interlocking in the form of input telegrams. For this purpose the modules generate special signals which replace the usual 50v d.c. feeds. Having passed through the relevant circuitry  these signals are returned to the module which converts them into the data required for the input telegrams. Two types of module are provided which can deal with all normal types of trackside equipment. 2.5.1 Points Module This module has been designed to drive directly clamp-lock type point machines although other types of machine may be controlled via interface relays. Up to 4 point ends allocated to a maximum of 2 point control numbers can be driven by each module, which also has provision for their detection plus inputs from other signalling fanctions. The points module contains 5 indication lamps which when lit indicate: Power module energised System module operational i.e. security fuse intact Rx Data own data being received Points X                   }                                   } module outputs operational Points Y                   }   X and Y is the nomenclature used to refer to the two sets of points which can be operated by each point  module. The point numbers are allocated to the X and/or Y module outputs at the design stage and will be found in the module schedule in the location diagrams. 2.5.2 Signal Module The signal module is designed to directly feed colour light signals and is also sufficiently flexible to cope with all signalling functions other than points. One signal module can cater for one or two signals depending on the complexity. It  has the facility  for lamp  proving and inputs from other signalling functions. This module has 4 indications similar to those mentioned above but only one "output" lamp is required to monitor the operational state of all module outputs. Data Link and Trackside Functional Modules 3. INTERLOCKINGS, PANEL PROCESSORS AND THE PANEL MULTIPLEX 3.1 Interlockings 3.1.1 Capacity of a Central Interlocking Unlike a relay interlocking, each SSI Central Interlocking (CI) has a maximum size in terms of signals, points, routes etc. These restrictions are due to the available memory size, addressing constraints and the processing power available in the multiprocessor modules. Several interlockings can be linked to overcome these restrictions. The capacity of a single central interlocking is defined by the interlocking memory all.ocation and the permissible Trackside Functional Module addresses. The limits are:- Signals 128 maximum; all types Points 64 maximum point numbers (single and/or double end) Track Circuits 256 maximum, including sections of a multi-section track circuite (e.g. a 2 section track circuit counts as 3 track circuites) Routes 256 maximum; a route is allocated for each slot or release. No routes are allocated for auto-signals. The maximum permissible number of routes could be a limitation in a complex station area. Trackside Modules 63 maximum. This is the most likely limiting factor. When designing a new interlocking it is normal to allow at least 10% spare capacity for future modifications Data complexity may also be a limiting factor, but this is unlikely, and calculation is a complex operation. It is outside the scope of this course. 3.1.2 Connection of Interlockings A signalling installation of any significant size will have more than one interlocking. The internal data link is capable of connecting up to 30 interlockings. This number is limited by available addresses but is large enough for all practical purposes. The boundaries between interlockings should be chosen on similar principles to those for relay interlockings, that is, to minimise the quantity and complexity of signalling data required to be communicated between interlockings. The ideal interface is at automatic signals. An SSI may control more than one area e.g.  one  interlocking controlling a complex station area and another controlling the simpler areas each side of the station. It should be noted that each interlocking should be totally within one signalman's control area (unless an IECC is used). 3.1.3 Interlocking Nomenclature Each interlocking will be given a name of up to 8 characters, typically an abbreviation of the signal box name and/or name of the interlocking area, e.g. LIVST1, LlVST2,  LIVST3 or LlVST3, LIVSTE, BETHGRN etc. The Interlocking will also be given an identity number (1-30) which is  unique  within  the signal box  or  control  centre. The identity number is included in the interlocking  data and is also wired into the interlocking MPM plug coupler. It is used to:- ensure interlocking MPMs are plugged into the correct cubicle. identify messages on the Internal Data Link. provide additional protection against cross talk on the Data links to the Trackside Functional Modules. (Where Trackside Data Links of adjacent  control centres are in close proximity the interlockings concerned must have  different identity numbers.) identify the interlocking to the Technicians Terminal. The interlocking data is also given a version number (0 - 255), which is included in the interlocking MPM data. A reduced form of the version number is wired into the  interlocking MPM connector, and  ensures that the correct version of data is contained by an interlocking MPM when plugged into the interlocking cubicle. The version number should start at zero when the Interlocking is first  commissioned and be increased by one at the commissioning of each subsequent alteration.  A  "sub-version" or "modification" number is also included in the data to distinguish  between intermediate issues produced during testing. The version and modification number system is also used to identify the different issues of other types of data used in the system (e.g.Panel Processor, Diagnostic etc), but it is not incorporated into the plug coupler wiring. 3.1.4 Interlocking Operation The interlocking MPM contains several processors each with their own program. All except the main processor deal with communication on the data links, coding and  decoding messages and checking for corruption. The main processor runs the functional program to · operate on the data. The main processor memory is divided into a number of areas. Program & Fixed Data which makes the MPM act as an interlocking. The same program is inserted in every interlocking module. Signalling Data - Fixed Geographical Data to configure the interlocking to a particular site. This includes tables of all valid signalling functions in the interlocking and the necessary interlocking logic (i.e. a translation of the control tables compiled in a form which the interlocking program can understand). State of the Railway - Stores the current state of every signalling function and is used in conjunction with the fixed data to determine whether a particular function can be operated e.g. signal cleared, points moved etc. Program variables -Working area for the main running program. On a cyclic basis, the interlocking performs the following activities:- Examines the incoming bits representing track circuits, lamp proving or point detection and updates the State of the Railway Memory. Examines the State of the Railway Memory for the various controls (say for a signal) and modifies the output bits if appropriate. Examines the Subroutes (route locking) to see if they may be freed to release a section of route locking.   While a delay in executing (c) would not lead to an unsafe situation, it  is obviously .  desirable to respond quickly to (a) or (b). The limitation to 63 trackside modules  (rather than 127 or 255) is a compromise between the length of time taken to  process the information and the response time acceptable in a safety system. State Of The Railway Memory Details BYTE BIT FUNCTION APPROXIMATE RELAY EQUIVALENT SIGNAL MEMORY 1 7 Train Operated Route Release - Approach Lock Free & No Train Approaching ALSR/TPR/TZR   6 Not red retaining     5 Route indicator lamp proved UECR (BR practice)   4 Signal stick SR   3 Aspect disconnected %   2 Temporary approach control %   1,0 Train past signal (approach lock release) ALSR (part) 2 7 Lamp alight ECR   6 Automatic (A)SR   5 Approach locking free ALSR   4 Signal aspect control UCR   3 Button pulled (FM)R down   2-0 Aspect code HR,HDR,DR 3 7-0 Approach lock timer (seconds) AJR POINTS MEMORY 1 7 Point Key (Lever) Normal (N)R   6 Points called normal NLR   5 Points detected normal NWKR   4 Points Key (Lever) Reverse (R)R   3 Points called Reverse RLR   2 Points detected Reverse RWKR   1 Points disable normal %   0 Points disable reverse % State Of The Railway Memory Details (continued) BYTE BIT FUNCTION APPROXIMATE RELAY EQUIVALENT ROUTE MEMORY   1 Route Set RUR (NLR down)   0 Route prohibit/disconnect %     TRACK CIRCUIT MEMORY   1 7 Track Circuite Clear TPR   6 Technician's control (set to occupied) %   5 Track Circuit Occupied TPR down   4 Not used     3-0 Track Circuit Clear Counter TPR delay 2 7-0 Track Circuite Timer TJR/TJPR FLAG MEMORY 1 bit only Sub route free USR Other functions (e.g. ground frame)   TIMER MEMORY 1 7-0 Other timers not included elsewhere POJR etc. % Restrictive control operated from technicians terminal 3.2 Panel Processor The. function of the panel processors in an SSI Interlocking is to interface with the Train Describer and with the Panel Multiplex system which drives the indicators on  the signalman's panel and detects operation of push buttons and switches. The panel processors are not part of the safety system and duplication is provided to achieve high availability. As with the Interlocking Processors themselves, the program in the Panel Processors is identical for all applications, and the logic required to control the panel indications and interpret push button and switch operations is stored in the processor's memory as tables of geographical data which are interpreted by the program. 3.2.1 Memory Areas a) State of the Railway The Panel Processor gets its information about the current state of the railway by  copying across areas of the Interlocking Memory. Each location in these parts of the Panel Processor is updated once in every interlocking major cycle of approximately  0.8s. To reduce the amount of data to be transferred, only parts  of  the  Interlocking  Memory are copied, so the Panel Processor contains a sub-set of the information in the Interlocking. b) Input Image The input image contains a copy of the information received in the last valid message from the panel multiplex, describing the state of the switch contacts in the panel. The Steady State Input Image stores the current state of every input, and an additional Active Input Image indicates that the signalman has operated a switch or button which may require action. The Input Image contains space for 256 contacts. c) Output lmage The Output Image contains the information which is sent to the Panel Multiplex to control the indications on the Panel. These indications are either flashable or non-flashable. Non-flashable indicators are grouped at the start of the Output Image and are allocated one bit each; if the bit is set the indicator is illuminated. Flashable indications are allocated two bits each and are grouped at the end of the Output Image ; the two bits are interpreted as a code to mean "extingushed", "illuminated-flashing", or "illuminated-steady". The total Output Image contains 192 bytes, giving space for 1536 non-flashable or 768 flashable outputs, or appropriate combinations of the two. Panel Multiplex - General Arrangement 3.3 The Panel Multiplex A Panel Multiplex System is required to convert the serial data stream transmitted from the Panel Processor into the drives to individual lamps or LEDS on a conventional signalman's panel, and to sample the state of all the panel switch contacts on a regular basis and make-up a serial stream for transmission back to the Panel Processor. · In each case the message corresponds to the contents of the corresponding memory area in the Panel Processor, with each byte converted into serial form and transmitted in address order. The messages also contain control characters which define the start and end of the message and a status byte which is used to monitor the correct operation of the panel multiplex. A further control character is used to identify the change from information for non-flashable to flashable indications. 3.3.1 Message Format The message format for in   puts and outputs is similar. Information is transmitted seqentially in one long message. Once the message is completed the cycle repeats. There is no provision for "Immediate Access". Information is grouped together in "bytes" of 8 bits each plus start, stop and parity  bits. The complete message contains a string of data bytes together with various control characters.   STX Status DATA DATA   DATA DCI DATA DATA   DATA ETX BCC         Non-flashing outputs   Flashing outputs     Next Cycle STX  -  Start of Transmission DCI   -  Data Control Character - end of non-flashing outputs ETX  -  End of  Transmission BCC  -  Block check  -  a further parity check  on the message The first byte of every message contains status information and checks the operational state of the Panel Multiplexer. It originates from the Panel Processor, appearing as the first byte in the output message. On receipt by the output card concerned it is fed to the first input card where it is modified before appearing as the status byte in the input message back to the Panel Processors to provide the checking functions. The status byte also contains information to drive the signalmans alarms "Critical", "Non-Critical" and "Normal Working Failed". The Panel Multiplex also drives the "Indications Failed" alarm and returns the "Alarm Acknowledge" button pressed to the Panel Processors. 3.3.2 The Westinghouse S2 Panel Multiplex System Each panel processor in the Central Interlocking cubicle is connected  via a current loop data link to one of the two scanner modules in  the  Panel  Multiplex  output  housing.  These  are both connected to all DOP cards via their respective  highways. The scanners accept the output data in serial form and transfer it to each  DOP in turn.         - As both panel processors are producing the same data, only one scanner is programmed to be "on line", with the provision to change over to the other under certain fault conditions. The two scanners in the input housing collect the data from the DIP cards before transmitting it serially to the two panel processors. Each DIP or DOP card caters for 32 Inputs or Outputs and requires 4 consecutive bytes of data (8 for flashable outputs) as shown below. The first DIP or DOP card is limited to 24 inputs/outputs to cater for the status bits. STX BYTE 0 STATUS BYTE 1 DATA BYTE 2 DATA BYTE 3 DATA BYTE 0 DATA BYTE 1 DATA BYTE 2 DATA BYTE 3 DATA     DIP/DOP CARD 0 DIP/DOP CARD 1   4. DATA TRANSMISSION 4.1 Introduction The Central Interlocking communicates with the Trackside Functional Modules via the Data Link cables. Commands from the interlocking are sent as output telegrams and replies from the trackside modules are termed input telegrams. The transmission method used is half duplex, controlled from the Central  Interlocking. Each trackside module is addressed in sequence from the central interlocking via the output telegrams. The interlocking waits for each module to  reply before sending out the next output telegram, however if no reply is received within a certain delay period the interlocking will output the next telegram and not wait for a reply. This means that failure to obtain a reply will not stop the system but  the trackside module in question must wait until the next time it is addressed via an output telegram before it can transmit its data. 4.2 Message Format The format for both input and output telegrams is identical. SYNC CLOCK RECOVERY DIR 1 BIT INTLKG IDENTITY 5 BITS MODULE ADDRESS 6 BITS DATA 8 BITS STATUS 5 BITS PARITY 5 BITS The first part of any message consists of Synchronisation and Clock recovery bit  patterns used to condition the receiving processors to be ready to accept it, and for the processor to improve its estimation of the message timing. This is followed by a direction bit abbreviated DIR in the format diagram which is set at "0" for an output telegram and "1" for a reply. There are then  two address sequences. The first being the interlocking identity  which is a 5 bit pattern and whose purpose is to ensure that each trackside module  only accepts data from its own interlocking. The second address sequence contains the unique address of the trackside module being serviced and consists of a 6 bit sequence which gives a maximum capacity of 64  addresses. Address 00 is assigned to the Diagnostic MPM which receives its information from the data link. The other 63 are allocated to the trackside modules. Following the address are 8 bits of data, 5 status bits and 5 parity bits. 4.3 Output Telegrams In output or command telegrams the status bits are not used and are therefore all set to "0". Also the data pattern varies between signal and point modules  i.e.  although 8 bits are allocated and can be used by signal modules, point modules can only control two sets of points therefore only 4 bits are required  (i.e. "X"  N, "X"  R, "Y"  N "Y"  R) for this purpose, the remaining 4 bits being held at "0". 4.4 Input Telegrams These messages carry indication and proving data back to the interlocking. The status bits, are used in input telegrams to provide certain monitoring information.  Reply message data and status information given is illustrated later. 4.4.1 Signal Module - Reply Message Bits 7 and 6 are used to provide lamp proving of signals and route indications and  bits 5-0 are available for other signalling inputs i.e. filament failure, track circuit indications etc. 4.4.2 Point Module - Reply Message Input telegrams from point modules always reserve bits 7-4 for the detection of  the possible two sets of points as shown in the diagram. The remaining 4 bits are available for other signalling inputs. 4.4.3 Status Bits Status bits are used in reply telegrams for both signal and point modules. Only 5 bits are transmitted but when printed on the technicians terminal appear as 8 bits of which bits 2-0 are not used and are usually set to "1". Bits 7 and 6 indicate the state of  the data links and should both be "1" indicating  that they are good. Bit 5 indicates the state of the module output interface i.e. the ability to drive the apparatus, and should be "1". If the output of a signal module fails this bit will change to "0" and the module will go into its RED RETAINING  state which  maintains the signal at danger. If it goes to "0" for a point module then it indicates that one or both of the point outputs have failed and are inoperative. Bit 4 indicates the state of the module input interface which is the input sensing, i.e. detection circuits. This bit again is normally at "1" and will change to "0" if the two microprocessors in the module fail to agree on the state of an input. This condition is likely to be caused by a module fault. Bit 3 is not used in a signal module and is set permanently at  "1".  In  a  point  module controlling clamp locks which require two outputs i.e. valve and motor, a failure within a module could energise the motor drive to a machine which was not requested by the interlocking. The detection of this situation would cause bit 3 to change to "0". Typical Reply Telegram - Data & Status Bits - Signal Module Typical Reply Telegram - Data & Status Bits - Points Module 4.5 Trackside Data Link & Data Link Modules Each leg of the basic trackside data link has a DLM in the central interlocking cubicle feeding a DLM in each location. To extend the data link, two DLMs  can  be connected in a "back to back" configuration to operate as a repeater on the trackside data link. Each DLM has two out puts to the data link ("Data Link L" and "Data Link R") which can be used to split the data link to feed separate branches according to the physical layout of the area. Trackside data links may take many forms. The following restrictions apply. For each data link section the distance between the two furthest  extremities  is limited to 8Km. At these two extremities a 100 ohm terminating resistor must be provided. Where no repeaters are connected this distance may be increased to 10Km. Each data link can be extended by using additional data link sections, linked by pairs of DLMs in back-to-back repeater mode, but the number of repeaters between the Central Interlocking and any extremity must not exceed four. Thus the maximum distance from the interlocking to the furthest extremity is five sections at 8Km, i.e. 40Km. Spurs may be provided but each spur should be limited in length to 1Km and the total length of the data link section and its spurs must not exceed 10Km. Spurs start at a simple T junction in the data link and if a spur's length exceeds that laid down in 1) or 3) then either a repeater should be provided in the spur, or a repeater provided at the junction. Any DLM, whether connected to TFMs, LDT or Central Interlocking can be placed in any position in a section. Thus a data link section does not have to be end fed by a Central interlocking Protection of DLMs against lightning strikes is provided at the cable input to locations. Various different types of circuits and devices are in use to provide the most effective form of protection in specific situations. In order to provide more helpful messages associated with faulty data links the diagnostic processor is programmed with details of the various branches  in  the data links, including the relative positions of all DLMs and TFMs. This information can most  easily  be visualised in the form of an analysis map. Data Link Analysis Map Typical Data Link Configurations 4.6 Data Link Cable The data link cable is 2 core solid conductor (2c .1/1.22mm) with red and blue insulation. The metal moisture barrier is not connected to earth since it is too light weight to provide immunisation in a.c. traction areas, but it does balance the cable cores relative to earth. The connections throughout the data link must be maintained in the correct phase as the data signal is polarity sensitive. Two versions of the cable are available, one has a thin sheath and is intended for use in cable troughing, the other has a thicker sheath and may be mole ploughed. Alternatively telecomms cable to BR Spec. 887 may be used. 4.7 Internal Data Link The internal data link is required when interlocking cubicles need to transfer interlocking information. The operation of this link differs from the external data link in a number of important ways. Up to 30 interlocking cubicles can be connected to one internal data link, which is arranged as a star network. The centre of the network is a pair of busbars which also have two 100 ohm terminating resistors connected in parallel across them. Each interlocking cubicle connected takes it in turn to put information onto the link. The message format is shown below for one such message. SYNC CLOCK RECOVERY INTLKG IDENTITY 5 BITS 15X8 bit BYTES OF DATA Addresses 64-78 120 BITS TOTAL PARITY 8 BITS Synchronisation and clock recovery are required as in the external data link in order to condition the receiving processors. This is followed by the interlocking identity of the interlocking sending the message (the same identity as used in the external data link). Then follows 15 blocks (bytes) of data each containing 8 bits, for convenience these 15 bytes are referred to as addresses 64 - 78. All other interlockings receive the 15 bytes of data but will only extract those bytes which they are programmed to take. So for example the message from interlocking "1" may consist of bytes 64 and 65 going to interlocking "2", bytes 66 and 67 going to interlocking "3" and bytes 68 to 70 going to interlocking "4". Each byte of data can be treated either as an 8 bit code (giving 256 combinations) used normally for route requests between interlockings, or as 8 individual bits to represent individual functions. The message is completed by 8 bits of parity which is the Hamming code and the whole message is Manchester II coded to give the same level of safety protection as is given to the external data link. The interlockings take it in turn to transmit their messages onto the internal data link, the order being set by the identity numbers of the interlockings. Toe data link has  been designed so that should one interlocking stop or for any reason fail to transmit its message the other interlockings can carry on and will not wait indefmitely for the missing message. 5. LONG LINE LINK & LONG DISTANCE TERMINAL 5.1 Long-Line Link The Long Line Link is used in place of all, or part of, a Trackside Data Link to extend the range beyond the usual 40km limit. It uses standard telecomms PCM systems to transmit the messages. A Long Distance Terminal (LDT) acts as an interface between the SSI equipment and CCITT G703 Contradirectional Interface. To provide the extra security needed in diversely routed telecomms systems, the  LDT adds a Control Centre Identity Number to each message as it enters the link, and checks and removes it as the message leaves the link. This Control Centre  Identity Number (between 1 and 2047) must be unique within a railway administration or more importantly within the whole communications network used for long line links. On BR the unique number is allocated by the headquarters office for each control centre or signal box. The Long Line Link  is duplicated, as with Trackside Data Links, and is connected  direct to the Interlocking via an LDT.  These two LDTs are mounted in the  interlocking cubicle in place of the DLMs for the Trackside Data Iink. A mixture of Trackside Data Links and Long Line Links both connected directly to the interlocking is not allowed. An LDT is then used whenever a connection to TFMs or, via a further DLM, to a Trackside Data Iink is required. 5.2 Range of Long Line Link The maximum length allowed for a Long Line Link is determined by the propagation delay through the telecommunications system from the interlocking LDT to the trackside LDT and will typically be in the range 560-780km when the trackside LDT is directly connected to TFMs only. Where the trackside LDT is connected to a  trackside datalink, the allowable Long Line Link delay must be reduced to take account of the time taken for messages to pass from the trackside LDT to the most remote TFM. It is often desirable to provide diverse routing for the A and B datalink paths through the telecommunications network. The only additional restriction imposed is a  maximum difference in arrival times of messages at any TFM. If there is no diversity in the trackside datalinks, this difference in path length through the  telecommunications system is 460-630km for typical equipment. Where the routing  of  the baseband datalinks differ, this will also have an effect on the difference in arrival time of messages at each TFM over the  two paths. 5.3 The Long Distance Terminal The LDT is mounted in the same size box as a DLM. It has six TFM connections  which can be connected according to the same rules as a DLT. Each LDT   is a  duplicated processor as it is modifying safety information. The 75 way plug coupler has the control centre identity wired in separately for each processor - as with the module address for the TFM. In addition to the 11 inputs to each processor for the control centre identity, a further input indicates whether the LDT is at the interlocking or remote from it. The power consumption of an LDT is 20VA. Long Line Link and Long Distance Ternminal 6. LOCATION CIRCUITRY 6.1 Introduction The various location circuits can be split into three definite areas: Power Supplies. Trackside Module Output Circuits. Trackside Module Input Circuits. 6.2 Power Supplies Each trackside module must be connected to a separate, isolated 110v a.c. supply to eliminate the effect of earth loops in certain failure modes. Each point module also requires a 140v a.c. supply which will be internally rectified to produce a 122v d.c. availability for clamp lock valve and motor drives. Where 650 volt power distribution is provided, a range of 650v input transformers is available:- 650/110v,   500VA for a single signal module 650/110/110v,  500/500VA to power two signal modules 650/140/110v,  4CL/100VA for a single points module (4 clamp locks) The location Data Llnk Modules may be connected to any 110v Trackside Module supply. Where more than one Trackside module is fitted, the Data Link Modules should be connected to separate supplies to improve availability. DLMs may alternatively be fed from a battery backed 12V dc supply. This is typically used for a back to back repeater location, especially if fed from a "non-guaranteed" supply. Other 110v 50Hz signalling equipment may be connected to a module power supply transformer subject to loading constraints, otherwise a separate transformer may be required. Power ratings (maximum) of SSI equipment are:- Data Link Module 110 V  50 Hz  7 VA   :  12VDC   7VA Single Module 110  V  50  Hz  35  VA  internal, plus external load Points Module 110  V  50 Hz  35 VA internal 140  V  50 Hz to external load --depends on points connected 6.3 Trackside Module Output Circuits 6.3.1 Signal Modules   All signals including automatic and semi-automatic signals are controlled by direct connection to signal modules. In addition other types of signalling functions can be operated e.g. BR AWS where a transformer rectifier or relay interface will be required. Each module can provide up to 8 controlled outputs (numbered 7-0)  at  110V  50  Hz  to operate various signal and route indicator arrangements. The most restrictive aspect of  any signal controlled by a particular module is connected to one of two  outputs (7  or  3) which have the facility to provide what is called a "red retaining" feed, which in the event of a module failure will energise to illuminate that lamp. Note that, as there are only two red retaining feeds per module, SRA double light signals will require a complete module. Signals with flashing (or pulsating) aspects can be catered for. They require extra  outputs in order to control the flashing.  When the signal is required to flash outputs  5 and 6 turn on and off to create the flashing while outputs 0 and 1 remain on to provide a low current level (via the resistors) to hold up the signal EKRs and  increase lamp life. Note, however, that in the output telegram bits 5 and 6 will be "1" for a steady or flashing aspect and bits 0 and 1 will be "0"  for a steady  aspect and  "1" for flashing. Bits 0 and 5 correspond to the top yellow while bits 1 and 6 correspond to the bottom yellow. Signal lamps which are permanently lit e.g. shunt signal pivot lamps and  L.O.S. signals are fed directly from the module fuse, termed the fixed source. All outputs from a module must have a return path to one of five current path connections (numbered 0-4). Current sensing (lamp proving) is provided on current paths 1-4, but current path 0 does not have current sensing and so is used for all output circuits not requiring lamp proving. The values of current proving on paths 1-4 are standard (as shown below), but may be reduced by adding an external resistor and modifying th e module strapping. Current Path      1 2 3 4 35W (1xSL35) 120W (i.e. 2x60W bulbs) 70W (PLJI) 35W (1xSL35) 6.3.2 Point Modules All motorised points will be controlled and detected via  the  point  modules. These  are designed to drive directly clamp lock mechanisms but, due to having only one point motor output per point end, will operate other types via interface relays. Each module can provide up to 8 d.c. outputs i.e. 4 motor drives and 4 valve drives  (2 normal and 2 reverse). To drive combined electrical machines the valve outputs  are used to operate the point contactor relays (N/RWR) over whose contacts the  point motor feeds are connected. Note the 1000 ohms 6 watt dropper resistors required to reduce the dc valve output to operate 50v relays. 6.4 Trackside Module Input Circuits Both signal and point modules are able to send information about 8 functions back to the interlocking by way of the input telegram. Each input will respond only to the  pseudo random binary coded signal generated within that module and output via he BXI/NXI and BXE/NXE supply connections of the module. These two input supplies BXI/NXI and BXE/NXE are identically coded but are electrically isolated from each other. BXI/NXI should be used for input circuits  internal to the location and BXE/NXE for input circuits external to the location. This  segregation of supplies allows vital internal inputs to be single cut. Each input will respond to either supply. As each input has BX and NX terminals then each can be either single or double  cut. Where single cutting is used, the BX circuit is taken via the relay or other contact and the NX terminal is linked to the input supply NX via a terminal strip to allow a  meter  to be used for testing. An input will detect an open circuit when the loop resistance is greater than 3.5 Kohms and will detect a closed circuit when the loop resistance is less than 500 ohms. Module inputs may be used to monitor contacts in another location, saving on the number of modules required. 6.4.1 Signal Modules Of the 8 inputs to a signal module bits 5-0 may be used to monitor general signalling  circuits e.g. TRs, first filament proving. Inputs 7 and 6 are used for lamp proving (Input 7 = current path 1, input 6 = 1 of current paths 2, 3 or 4). 6.4.2 Point Modules Of the 8 inputs to a point module only bits 3-0 may be used for general signalling inputs. Bits 7, 6 are used to monitor the detection of the "Y" points and bits 5, 4 are  used to monitor the detection of the "X" points. It was originally found that point modules monitoring detection were able to detect momentary breaks in detection caused by vibration of trains passing over the points. These would be unnoticed with a normal relay circuit. In order to avoid problems  from this effect the detection inputs have been redesigned with the equivalent of a  "slow to drop relay" feature thus preventing slight vibrations from causing loss  of  detection,  momentary replacement of signals to danger and unnecessary fault messages. The usual detection arrangement is for the special input signals BXE,NXE to be passed through the detection springs in the machine directly and then fed back into the relevant detection inputs on the module. The down proving of the contactor relays will be achieved by one of the general purpose inputs on the point module. Typical Signal Module Wiring BR 4 Aspect Signal with Junction Indicator, Position Light Subsidiary and AWS Typical Signal Module Wiring for Flashing Yellow Aspects Typical Points Module Wiring Two Double Ended Clamp Locks Point Module Outputs for Point Machine Operation Typical Trackside Module Input Circuits Typical Point Module Inputs (Clamp Lock Detection) Typical Point Detection Circuits for Electric Point Machine 7. THE TECHNICIANS TERMINAL 7.1 Introduction The provision of a diagnostic aid for fault finding has been considered an essential part of the SSI system. Its purpose is to interpret the messages transmitted between the various parts of the system and thus largely replaces the process of tracing voltages in circuits with a voltmeter. Additional facilities include the ability to apply restrictive controls to the interlocking. A magnetic tape record of changes to signalling inputs, outputs and panel requests is provided. The technician's terminal contains a specially configured multiprocessor module called the "Terminal Processor" and this is used to control the other parts of the technician's terminal which consists of a printer, a VDU, a keyboard, a data recorder and a modem link. A printer is provided to give a permanent record of all fault messages, it can also be used by the technician to provide information for fault finding.  A VDU is provided  in addition to the printer, the printer normally only prints out the fault messages and the VDU is used by the technician. The modem is included so that a technician's terminal, remote from the  interlocking, can be used in much the same way as the local terminal. This could be useful if  second line technical support staff are located some distance from the interlocking. The tape cassette data recorder provides a record of all changes in the system including signalling input and output messages, panel requests, faults, and control alterations from the keyboard. This could provide useful historical information in the event of an incident. Each tape drive is used alternately, previous data being overwritten. If it is required to retain the data, the appropriate tape cartridge must be exchanged. One technician's terminal has the capacity to serve up to six interlockings, but a separate diagnostic processor is required for each. 7.2 General Information Each line of output printed starts with a four digit sequence number which is automatically incremented. In addition the date and time are printed with certain  messages, this can   be in one of two forms. A. (date) (time) e.g. 05.03.85 12.23.36 GMT B. (time)            e.g. 12.23.36 7.3 Fault Messages For most of the time the terminal will not be required for information or commands  and instead the printer will be available to print out failure messages as they are detected by the diagnostic processor. Failure messages in general consist of two lines. The first line consists of the following: Sequence Number Time Message Type including Interlocking Identity The interlocking identity is the eight character abbreviation of the name of the . interlocking from which the message originated as used in the geographic data for that interlocking e.g. LSPA. In the case of messages originating from the technician's processor itself, the name TECHNICIANS TERMINAL is used. There are four classes of fault message, each of which has a special group of symbols to make class identification easier and quicker. They are as follows: Class of Fault Fault Symbol FAULT !!!! INTERMITTENT FAULT %%%% FAULT CLEARED **** ALARM @@@@ A fault is declared intermittent if it has occurred, cleared and then occurred again as shown in the example below. No further messages or alarms will occur once a fault has been declared intermittent until the fault entry is REMOVED from the fault list . The second line consists of the following: Sequence Number Failure Description The following are a small selection of the messages that could be produced. a) 5515     13:34:08    !!!!   FAULT ON LSPA      5516                             SIGNAL 41 RED LAMP A total lamp failure has occurred in 41 signal red aspect at 13:34. b) 5819    17:10:44  **** FAULT CLEARED ON LSPA     5820                          NO REPLY. TRACKSIDE FUNC. MODULE 10 BOTH LINKS A previous message would have stated that the interlocking was not receiving data from TFM 10 on either data link. This fault has now cleared. C) 5552    13:45:53   %%%% INTERMITTENT FAULT ON TECHNICIAN'S TERMINAL      5553                                    DATA PANEL I/P 0 On at least two occasions the technician's terminal has received corrupted data from the PPM link on I/P port 0. d) 5771 17:09:10 @@@@ ALARM ON LSPA      5772                  DIAGNOSTIC PROCESSOR OUTPUT BUFFER OVERFLOW The diagnostic processor has been unable to cope with all the fault messages being generated. e) 5508 13:32:08 !!!! FAULT ON LSPA      5509                     MONITORING LOC106.4ELK EARTH LEAK WARNING An earth leakage current has been detected  at location  106.4. This message  is an example of how specialised messages can be added to the standard messages and generated when a specific change of state occurs. 7.4 Information Requests Before requesting information from the terminal it is necessary to electronically unlock the keyboard by typing in a four digit combination code, preceded by . The code will not be printed on the terminal. If an incorrect code is entered, the terminal will print. INCORRECT CODE (time) KEYBOARD LOCKED If the correct code is entered, the terminal will reply KEYBOARD UNLOCKED and will then print the current terminal configuration followed by the display mode prompt which is an arrowhead symbol ">". The terminal must be configured to accept information from specific interlockings connected to its six ports, this configuration is printed when the keyboard is unlocked as a reminder to the user. An example printout is shown below. 0484 KEYBOARD UNLOCKED 0485  0486 IDENT NAME PORT 0487 0 0488  1 0489   2 0490   3 0491   4 0492    5 0493   0494   > To LOCK the keyboard, type Lock and the terminal will reply (time) KEYBOARD LOCKED If the terminal is left unused for 30 minutes unlocked, it will lock the keyboard automatically and print the keyboard locked message. The complete list of requests, commands and controls and their operation is not included. However, a summary of the main facilities is given to give an appreciation of the use of the technician's terminal. 7.4.1 Fault Summary To obtain a listing of current faults, type FAULT terminal will then print a numbered list of faults If a fault occurs and is subsequently cleared, its status in the list will change from  "fault" to "fault cleared", but will remain in the list. If the fault occurs again before the "fault cleared" has been removed from the list, its status changes to "intermittent fault". 7.4.2 System Activity To display system activity on the printer, type DISPLAY (name) (device)............(device)  (name) is the interlocking code name and (device) is the device type to be logged,  as follows: TRACK for track circuit indication changes POINTS for points indication changes SIGNALS for signal indication changes One or more device types may be specified, separated by commas. The terminal  will then start to log activity with the relevant time. 7.4.3 Telegram Contents To display the contents of specified telegrams, type TELS (name) (address 1)...........(address 4)  Name is the name of the interlocking, and address 1 etc. are the addresses of telegrams to be examined. Up to 4 addresses may be specified. The terminal will immediately reply with the current state of the specified telegrams, and will printout each telegram contents again whenever it changes. Each telegram description occupies one line and takes the following form. TELS  LSPA  22.23 0507  09.21.53  AD OUT DATA   IN  DATA   OUT  STAT  IN  STAT 0508                         76543210   76543210   76543210    76543210 0509  09.21.58  22   10001000    10001000   00000000    11111111 0510  09.22.00  23    10000000    10001000   00000000    11111111 0511   09.23.23  22    10000001 0512   09.23.23   22   10001000 0513   09.24.23   22    10000100 0514   09.25.23    22   10000001 0515   09.25.23     22   10001000 0516   09.25.50     23   00100100 0517   09.25.51     23                         11001010 0518   09.26.03     23     10000000 0519   09.26.04      23                             10001000 0520  > OUT means from  the interlocking to the module. IN  means to the  interlocking from the module. DATA means interlocking information, i.e. controls or indications. STAT means the check that the module is operating correctly. Eight bit binary numbers are printed in the order bit  7, bit 6 .......,  bit 1,  bit 0 When the data and status are first printed, all four sets of information are displayed on one line. When a change occurs, only the particular part containing  the changed information is displayed. The printout continues until terminated by typing 7.4.4 Control List To obtain a list of all controls currently set on a specified interlocking (e.g. route disconnections), type LIST (name)  terminal will then request the interlocking to send a list of all controls currently set. 7.5 Interactive Commands As well as displaying data and information necessary for general maintenance and fault finding it is possible to send commands to the interlocking in order to change various parameters. It is necessary to leave display mode, which can be thought of as the lowest mode of operation, and enter "command" mode. To enter command mode, type COMMAND    The terminal will reply COMMAND MODE prompt will change to "*" to indicate that the terminal expects interactive control commands. To return to display mode type 7.5.1 Clearing Fault Records Most types of  fault  registered in the fault table will automatically be changed  from  "fault" to "fault cleared" status when the fault has been rectified. However some faults, particularly intermittent faults, require this change of status to be done manually. To clear a fault record, type CLEAR n n is the number of the fault to be deleted. The  terminal will then print out the fault  message concerned and ask for confirmation. When confirmed the terminal will print FAULT CLEARED. 7.5.2 Removing Cleared Faults To remove all "Fault Cleared" status faults from the fault table, type REMOVE The terminal will then reply ALL CLEARED FAULTS REMOVED The table will then be compressed, and a new "FAULT" command will be needed to establish the new numbers of the faults remaining in the list. 7.6 Interlocking Controls Before any alterations can be made to the interlocking it is necessary to enter the highest mode of operation, "control" mode. To do this type CONTROL  The terminal will reply CONTROL MODE. ENTER NAME AND PURPOSE The user must then type in his name and the reason for altering the controls. This will be recorded as part of the logged data on the tape cartridge. The entry may occupy more than one line. Therefore, to terminate this entry type twice. The terminal will interactively ask for the interlocking name and the control required. There is a choice of controls which may be applied, these are ASPECT DISCONNECTION TEMPORARY APPROACH CONTROL TRACK CIRCUIT OCCUPATION ROUTE PROHIBIT POINTS DISABLE START (see 7.7) STOP (see 7.7) Enter the control required (only first four characters necessary). After the "*" prompt enter ON or OFF to apply or remove the restrictions, followed by the device reference number for the device to be controlled, obtained from the interlocking reference manual. ON and OFF commands will be coded by the terminal processor, and sent via the panel processor to the interlocking. Typing only will cause the terminal  to ask  for another control. If an ON or OFF command was issued, the interlocking will reply to the terminal, which will print out the identification of the device controlled as it appears on the control panel, and not as entered. If an incorrect command was entered, an error message will be printed. If the command was valid, the terminal will ask ENfER EXECUTION CODE*, the word "EXEC" must now be entered which will cause the terminal to reply COMMAND EXECUTED.If an incorrect command code or no code is entered it will print CONTROL NOT ACCEPTED. This process may be repeated as necessary. The following is an example of how to disable points 231 so as to make the field sidings out of use. Normal print shows the text generated by the terminal and bold text is the user input. 0538    COMMAND 0539    COMMAND MODE 0540   *CONTROL 0541    17.09.86   09.31.24   BST CONTROL MODE, ENTER NAME AND PURPOSE 0542    *M.SMITH. TAKE FIELD SIDINGS OUT OF USE 0543      0544     0545     ENTER INTERLOCKING NAME *LSPA  0546     ENTER CONTROL REQUIRED *POINTS  0547     POINTS DISABLE *ON  11 0548      POINTS DISABLE 231 ON 0549     ENTER EXECUTION CODE * EXEC  0550     CONTROL EXECUTED 0551     POINTS DISABLE *  0552     ENTER CONTROL REQUIRED *    0553     ENTER INTERLOCKING NAME *    0554     *  0555     > LIST  0556     NO SUCH INTERLOCKING 0557     > LIST LSPA    0558     POINTS DISABLE 231 0559     END OF LIST 0560  > 7.7 Power Failures In the event of a complete power failure to the interlocking one of two sequences will occur depending on the length of time the power is lost. The interlocking MPMs compare memories and decide on the type of start to adopt. 7.7.1 Cold Start If the power is off for more than approx. 6 hours the interlocking will have lost all the information in the State of the Railway Memory including any restrictive controls. Before normal operation can continue it is necessary to start the interlocking from the technician's terminal. This is done in control mode. A 4 minute time-out period and the START command must have occurred before normal operation can resume, however the START command may be given at any time after the power has been restored either during or after the timing out period. Any controls applied before the power failure will have been lost and MUST be re-entered before giving the START command. As 6 hours may elapse before a cold start is necessary, it should never be required under normal circumstances. 7.7.2 Warm Start If the power is off for less than approx. 6 hours then data will not be lost and no action will be required by the technician, however the signalman will have to reoperate any routes previously set. 7.7.3 Four Minute Tune-out Occasionally after a warm start, a discrepancy in route set memories may be found by the Interlocking MPMs during memory comparison. This can occur if a route was being set or released at the instance of power failure. In this situation, a modified warm start will occur where all routes are released but points are immovable, signals held on, and signalman's controls ineffective for a four minute time-out period. All train movements should have come to a stand in this period. This timing out is indicated to the signalman and on the technicians terminal. The timing out period also applies to every cold start. 7.7.4 Stopping an Interlocking The STOP command can be used to stop an interlocking to enable a large number of controls to be set or cleared. In the stopped state, an interlocking will still report the positions of all trains, but all signals will be held at red, and all points will be locked in their curent position, and the signalman's controls will be ineffective. 7.8 Data Link Interogator A small portable unit is available which can be connected to the data link (normally at the line termination unit) which can be used to display the individual messages sent between the interlocking and the TFMs. The information displayed will be exactly the same as the information given on the technician's terminal using the TELS command. 7.9 Go/no go Tester When it is anticipated that a point or signal module may have to be changed it is important that any replacement module taken out to the location is known to be working.  A  GO/NO GO test unit may be available to test point and signal modules quickly and simply before leaving. Detailed instructions on how to operate the unit may vary but it will consist basically of plugging the module into the tester with the appropriate connector plug and then starting  the automatic test routine. Simple indications will then indicate when the test is complete and whether the module is good. Using the GO/NO GO tester will deconfigure the module of its interlocking identity to allow it to be moved from one interlocking to another. 8. SSI DESIGN & TESTING SSI requires a fundamental departure in design principles from relay interlockings. Instead of designing physical circuits, much of the information must be coded into stored computer data. The SSI design workstation has been developed to enable designers to prepare and test geographic data for SSI signalling projects without a detailed knowledge of the computer languages used. The design engineer will never need to amend the SSI programs; in fact the workstation does not give him the facility to do so. 8.1 The Design & Test Process The stages involved in the design and testing of the geographic data for an SSI signalling project are summarised on page 55. The processes on the left half of the figure are basically manual procedures which have direct equivalents in the design and testing of conventional signalling schemes. Some of these processes make use of the Design Workstation as a means of storing information and of carrying out tests in the design office rather than on-site. The processes on the right half of the figure are mainly automated procedures carried out within the Design Workstation. 8.2 The Design Process Signalling plans and control tables must be prepared in the same manner as for conventional installations. The adoption of SSI has resulted in some changes to the format of control tables as it is important that the information from which  the interlocking data is compared is complete. Many control tables have in the past made certain assumptions regarding standard controls and these have not been shown. It is important that the data preparation engineer has all required controls documented before input starts. The first major task, once plans and control tables are available, is the allocation of all items of lineside equipment to trackside modules. Each function must be provided with the required number of inputs and outputs. In general, point operation outputs must be provided by points modules, all other outputs will be provided by signal modules. If the overall number of trackside modules exceeds the capacity of a single interlocking, the overall control area must be divided between a number of SSI Central Interlockings. The boundaries should be determined so that the data passing between adjacent interlockings is minimised. A section of automatic signalling is ideal although this will not always be possible. The number of trackside modules is the most likely limit on the area of one interlocking, although other constraints such as the number of signals, routes, points or track circuits (or even the total ·volume of data - not yet known - could be a limitation. The module allocation should also include the suitable positioning of data link modules as repeaters. Any one interlocking can only operate with the standard data link or the long distance terminals, not a mixture of both. This is due to the different communications software used. The module allocations will be stored on a database within the Design Workstation,  which is capable of generating printed listings in a variety of formats. The next task is Data Preparation. This is the specification of the geography of the railway and all necessary signalling controls in a form which the workstation compiler can understand. It is still in a user readable form (source code). From this data, the SSI workstation will compile various data files for use by the·  interlocking modules, the panel processor modules, the diagnostics and the simulator for testing. 8.2.2 Data Preparation The SSI system must know the identity of all functions which it controls or may receive inputs from. A series of identity files lists all functions. If a function does not appear in the identity file, the interlocking software will not recognise it. The identity files for interlocking XXN would be:- XXN.TCS Track circuits XXN.SIG Signals XXN.PTS Points XXN.ROU Routes XXN.FLG Flags (any other two-state inputs, outputs or data required by the interlocking - e.g. Train ready to start indicators and ground frame release, also includes sub-routes and sub-overlaps XXN.ELT Elapsed timers (other than signal or track circuit timers which are already assumed to exist for each route and track circuite) XXN.QST Route requests XXN.BUT Panel switches and buttons XXN.IND Panel indications SSI Design and Data Preparation Other files will determine how the interlocking operates. The interlocking requires the following additional files:- XXN.IPT All input functions from the trackside modules (external data link) and other interlockings (internal data link) XXN.OPT All output functions, internal and external XXN.FOP Flag operations. This includes route releasing of sub-routes XXN-PRR Panel requests. This requires all the conditions for setting of a route or a set of points and will contain the equivalent of the direct locking and the route locking in the signals controls  XXN.PFM Points free to move - all point locking controls - equivalent to the WZR in a realy interlocking although somewhat simpler since all locking on points is applied by sub-routes and sub-overlaps XXN.MAP Geographical map of track layout used for detecting approaching trains in the approach lock release function The panel processor modules require the following files:- XXN.PSD Signal indication data XXN.PPD Points indication data XXN.PTD Track and route indication data XXN.OTD Other indications data XXN.PBK Panel button and key data The diagnostics MPM will also require file XXN.DIA which is a map of the data link to allow the diagnostic processor to deduce faults from the data flowing (or not flowing) on the data links. Some of the data will also be used by the simulator. At this stage, all data is held by the workstation. When testing is satisfactorily completed, the data can be loaded on to EPROMs for installation into the modules on site. Files can be listed in a variety of formats to enable checking to be carried out. 8.2.3 Testing A major difference between SSI and conventional interlockings is that a large proportion of the testing can be done on the design workstation. Most of the functional testing of the interlocking is done in the office. The simulator is used in place of the data link and trackside modules. The tester has the facility to operate a simulation of the signalman's control panel and change the state of·all inputs to the interlocking. The simulator consists of a simplified SSI cubicle, containing only one interlocking and panel processor, the simulator MPM connected to its VDU and an interface processor connected to a mouse or trackerball input, communications to the main workstation computer and a tape cartridge unit. This can be used to read the data held on tapes retrieved from working interlockings for investigation. Testing on site can therefore be confined to the communication between the interlocking and the trackside module inputs and outputs. The logic of the interlocking does not need to be re-tested. It is also possible to prepare and test interlocking data for several stages of a multi-stage job in advance. The EPROMs are exchanged at the time of the alteration: 8.2.4 The Checking Process Just as wiring diagrams must be independently checked against the signalling plan, control tables and other source documents specifying the signalling requirements and controls, so must the SSI data. Some of the checking processes can be automated. Others still require a systematic manual check. The final check must always be on the data which is used by  the SSI, not on the source code input during data preparation. This can be checked for equivalence by a process of decompilation. The object code is converted back to the source code and compared with the original. This checks the. compilation process and can be fully automated. Checking that the data corresponds to the control tables and to signalling principles and standards is not yet an automated process. The engineer responsible for checking must inspect all the data to ensure correlation with the written controls. Although not automated, the task can be made easier by the production of listings of various parts of the data files arranged in a specified order. This will enable the checker to find all references to a particular function. Different versions of data can be compared to identify all differences. This assists the checker in ensuring that intended modifications have been carried out. As the control tables are essentially written documents, it may prove possible in the future for control tables to be prepared in a suitable format for direct compilation into SSI data. This would further simplify the checking process. The final manual check would be on the control tables themselves. The remainder of the data checking could be done by decompilation. If the control tables were prepared as a database,  it might also be possible to identify automatically whether converses of each control had been entered. 8.3. Analysis of Tapes The SSI design workstation is equipped with a tape cartridge reader which can be used to examine the contents of a tape. This is a useful aid for examining failures and incidents where allegations of irregular operation of equipment or movement of traffic is alleged. By monitoring all the input and output telegrams, the tape holds a complete record of the time of every change of state of the equipment. Typical Design Workstation Configuration 9. The Future SSI has already had a major impact on the control of signalling.  It  is  however  unrealistic to assume it is the final stage of signalling  development.  Technology  will  continue  to improve and one day SSI  in its present  form  will  no doubt  be obsolete. For the present, it is unlikely that any major changes will take place in the interlocking software. As so much effort is required in the development and validation of safety  software, significant changes have to be justifiable in financial terms. At the trackside, however, major changes may have to be accommodated. SSI has  already been employed successfully in the control of ATC driven trains. This  has, however required an additional interface between the trackside module and the ATC equipment. With greater interest in automatic train protection, a system which could interface ATP transponders and coded track circuits directly with SSI trackside modules would be a great advantage. Track circuits are already on the market which can drive a trackside module input without the need for a track relay. Train detection has traditionally been done by track circuits. With the increasing  interest in axle counters, a trackside module which could receive the input from the rail mounted equipment is under development. The counting logic would be carried out within the central interlocking. As well as reducing equipment complexity, and  hopefully  cost,  this  would extend still further the range of the axle counter.  Suitable protection will have to be provided against false operation due to timing of telegrams. As yet, no vital data link exists between SSI installations at different sites. The internal  data link is designed for communication between interlockings in the same  building.  With the more widespread application of SSI, the need for such a link increases. At present, the only means available for conveying data such as aspect sequences across boundaries between SSI installations is a back to back arrangement of trackside modules.      

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Posted 100 days Ago

Six cyber risk mitigation strategies for software obsolescence in railways

    Serge Van Themsche, Vice President of Strategic Partnership at Cylus, shares his six recommendations for the railway industry to avoid software obsolescence pitfalls.   Railway and public transport operators today are confronted with major obsolescence issues, a problem that will only grow with the increasing usage of commercial off-the-shelf (COTS) and Internet of Things (IoT) products. It is easy to understand why. The expected life span of rolling stock and other railway assets – which is from 20 to 40 years – collides with the much shorter lifecycle of COTS hardware. The utilisation of commercial firmware and Operational Systems within the railways’ Operational Technology (OT) environment and its hard-to-manage related software obsolescence only amplifies the problem.   Concretely, it means that, even with the best obsolescence management system, applying all of the recommendations from the standard IEC standard 62402:2019 1 , there will be a time when the rolling stock, signalling or any other railway sub-systems (e.g. SCADA, Passenger Information, Platform Screen Doors, etc.) will have to be operated with known obsolete elements. In fact, the International Association of Public Transport’s (UITP) cyber-security sub-committee working group on obsolescence 2  has identified that, after eight to 10 years, public transport operators must generally put in place mitigation measures to protect systems that inevitably become obsolete. This working group, that  Cylus  led, wrote a comprehensive whitepaper defining a strategy for public transport operators on how to deal with software obsolescence. Let me be clear: Obsolescence goes beyond  cyber-security  risks. Obsolescence creates sustainability risks, with significant impacts on operations and maintenance costs, as well as operational efficiency risks linked to RAMS (Reliability, Availability, Maintainability and Safety) issues are also involved. For those interested in these non-related cyber risks, I recommend consulting the IEC standard 62402:2019 3 , especially the section on risk assessment of obsolete assets and this UITP report.   Why can obsolete software become a time-bomb? Here are the reasons why operators must tackle obsolescence from a cyber-security perspective: Legal and regulatory compliance risks Operators relying on obsolete solutions can face heavy fines and even legal action if they do not comply with government or industry regulations, particularly when a data breach occurs resulting from the use of older technology with known vulnerabilities. No security patches Hardware obsolescence can be the triggering factor of firmware obsolescence, since no security updates are made. Without patches sent, systems are becoming vulnerable to known attacks that could be easily prevented. Discovery of new vulnerabilities Software (i.e. operating systems, firmware, application software) obsolescence on its own makes a rail network more vulnerable to cyber-attacks. As time goes on, the probability of finding new vulnerabilities only increases. Increased likelihood of exploitation The more vulnerabilities that are found, the greater the chance of exploiting them. To make matters worse, in the longer-term, low-skilled attackers can slightly adapt an already developed malware from other verticals and replicate the attacks. Not having to develop from scratch the attack vectors will only increase the attacker’s pay-back motivation and the likelihood of attack. Excluded from security ecosystem To aggravate the vulnerability’s impact, the obsolete software never really integrates the latest security controls coming from newer ‘secure by design’ coding best practices, making the detection more difficult and the exploits more likely. Furthermore, the newer protections offered by antivirus and similar  cyber-security  solutions are not tuned to malware attack signatures on outdated rail systems, once again increasing the difficulty to detect such attacks and the probability that it will happen. In other words, software obsolescence can become a time-bomb without the right cyber mitigation measures. Recommendation to mitigate obsolescence risks Based on my experience and the work performed within the UITP cyber-security sub-committee, here are my six recommendations to avoid obsolescence pitfalls: 1. Obsolescence planning Obsolescence monitoring should start at the tender stage, with requirements to be integrated within the system design phase and carry-on throughout the system’s entire lifespan. All railway and public transport operators should establish an obsolescence management system that follows the IEC 62402:2019 standard, which demands planned obsolescence risk assessments. 2. Asset monitoring and obsolescence identification Within their obsolescence policy, operators must map all their assets and identify when they are becoming obsolete. Though some follow-up can be done manually, with time passing by and the number of assets increasing, software driven monitoring solutions become mandatory. Monitoring systems with auto-discovery functionalities not only identify all assets running on the network, but are increasingly able to detect the hardware details with its software or firmware version and flag-out obsolete versions and their overall risk scoring. After setting a baseline, they also monitor any suspicious dataflow or unauthorised access attempts going to an obsolete equipment from an existing or new equipment. 3. Zone partitioning according to security levels The monitored assets shall be assigned to consistent security zones and policies connected by conduits according to the new TS 50701 4  standard (railway adaptation of IEC 62443) and based on an initial risk assessment. Asset partitioning should be possible according to the technology lifecycle (i.e. obsolescence) criterion, alongside the many other permitted segmentation criteria (e.g. risks of the asset in terms of: Integrity, availability and confidentiality; physical or logical location; access requirements; operational function; safety aspects, etc.). Modern continuous monitoring technology of OT networks allows for such partitioning and alert when these policies are being violated. 4. Treatment of obsolete IT  solutions The NCSC (the UK’s National Cyber Security Centre) recommends that obsolete systems should be treated as ‘untrusted’. It even recommends using only solutions still supported by vendors, which implies migrating away from obsolete platforms and applying short-term mitigations until this migration is complete. While applying this recommendation in IT environments is possible, it isn’t feasible for practical and economic reasons in OT rail networks. 5. Dealing with obsolete solutions in OT environments Implementing a monitoring solution that includes network traffic analysis and deep packet inspection capabilities is the only efficient mitigation measure besides physically isolating the network that is running the obsolete asset through a data-diode, a solution that generates many other complications in existing OT networks (e.g. latency, homologation). Just to emphasise once again, a monitoring solution is essential for asset supervision, obsolescence identification and zone partitioning, and can provide compensating controls to ensure real-time detection of malicious behaviour until these systems are migrated.    

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Deepu Dharmarajan -
Posted 90 days Ago

SIGNALS AND TRAINSTOPS

  CONTENTS 1. Introduction 2. Main Signals 3. Shunting Signals 4. Route and Junction Indicators 5. Repeater and Indicator Signals 6. Fixed Distant Signals 7. Emergency Signals 8. Trainstops 1. INTRODUCTION Lineside signals are the vital link between the safety controls of the signalling  system and the train driver. Although various forms of cab signalling, automatic train protection and ultimately automatic control are available and are being further developed, it will be a long time before all lines and all trains are suitably equipped. It is likely that many low density lines will never justify the provision of automatic train control. The failure of a driver to observe a signal could lead to an unsafe situation arising - a train might collide with another or pass over a junction at excessive speed. Even if no collision arose, damage could be caused to points and other equipment. Signals must therefore perform consistently and reliably. They must always fail safe. Generally, signals are of two types; main running signals and shunting signals. The requirements for each are significantly different. This section will also cover trainstops. Although a distinct item of equipment, their application and circuits are closely associated and it is difficult to separate the two. 1.1 Basic Requirements for Main Signals Main signals are observed by drivers of trains running at high speed. They must be capable of long distance sighting sufficient to give the driver time to react. In addition, trains may have to stop at a signal. Although less critical, the signal must still be visible at short range. Junction signals have to provide the driver with additional information on the route  he is to take. Whether this is done by additional aspects or route indicators to  supplement the main aspect, any route indication must have comparable visibility to the main signal to avoid misleading the driver. Precautions must be taken to detect the failure of a lamp in a main signal. A train  must not be allowed to approach a signal without a light in it, unless he has been  given suitable warning beforehand. 1.2 Basic Requirements for Shunting Signals Shunting signals are only used to control low speed movements. The construction  and optical systems of shunt signals may generally be much simpler. They need  only  provide for short range visibility. The risks associated with lamp failure are  much lower as movements are at low speed and shunt signals do not generally  convey information about the signal ahead. They are often mounted at or near ground level. 2. MAIN SIGNALS  2.1 Aspects Early signals were mostly semaphore signals. Even before electric power was  widely available, it was realised that a light indication was necessary ay night time. Colours were therefore used to distinguish between the different positions of the semaphore arm at night. The light source was at that time an oil lamp, with limited intensity and the tendency to deposit carbon on the lamp glass. Colours were used to give distinct indications but the choice of colours was limited by the variation of lamp output. Red and green spectacles were placed in front of  the lamp to give stop and clear indications. Blue was not used because it could be mistaken for green. Blue light, being at the high frequency end of the optical spectrum has poorer penetration through rain and fog. Yellow and white were not used at this early stage because most oil lamps  produced  a slightly yellow light. A broken or missing spectacle could therefore give a less restrictive indication. For this reason, the application of white lights must still be restricted today. Early signal engineers therefore had a choice of only two colours, red and green. Red was already universally accepted as a danger signal. Green was used for clear. The problem arose of how to distinguish between the caution aspect of a distant  signal (which could safely be passed) and a stop  aspect  (which could not). Some  railways relied on the driver's knowledge to distinguish between stop and distant signals. Others provided additional indicators to identify distant signals. Others  decided to provide more than one light - the relative positions of the lights would  indicate whether the signal was a distant or a stop signal. When electric colour light signals were introduced, engineers now had the ability to produce a much better quality of white light. Yellow became available as a third  colour. The problem was now whether to change the appearance of signals to the driver or whether to keep the set of aspects with which the driver was familiar. At about the same time, the need arose for an additional aspect to the basic stop, caution and clear indications. Around the world, there are therefore several different solutions to the same basic problem. Most British railways adopted the convention of red for stop, yellow for caution and green for clear. At junction signals, the same aspects are used for both main line and turnout routes. In New South Wales, the convention of using two lights was already firmly established. Colour light signals were therefore introduced with two heads, the upper head as the stop signal and the lower head as the distant for the signal ahead. Red and green could therefore be used in both without confusion. Yellow was initially used for junction signalling (until the need arose for a fourth aspect). Single light signalling was a later development designed to economise on equipment by providing only one signal head. Yellow was used for caution. Initially there was no preliminary caution or medium aspect. This was added later by pulsating the yellow light. We therefore have three different examples of a four aspect signalling system, all of which function satisfactorily but appear differently to the driver. Indication Double Light Single Light British Stop Red/Red Red + Marker light Red Caution Green/Red Yellow Yellow Medium or Prelim Caution Green/Yellow Pulsating Yellow Double Yellow Clear Green/Green Green Green 2.2 Junction Signalling On a signalling system which tells the driver how far ahead his route is clear, it is vital to tell the driver which of the available routes he is to take at a junction. Again, there is more than one way of solving this problem. The most common methods are:- Provide more than one signal - this is the usual solution with semaphore signals but produces an arrangement of signals which is complicated to construct and may be difficult for the driver to read. Some early colour light installations did provide more than one signal head but most engineers (and drivers) prefer a single signal head. Provide an additional aspect (or aspects) in the main signal to indicate that the turnout route is set. This solution is restricted to very simple junctions due to the available colours and the need to restrict the number of lights in a signal which are illuminated at any one time. Use the main signal aspect but qualify this by some form of route indication. The route indication may be provided for all routes or for the turnout routes only. In double light signalling, the use of a yellow turnout aspect is basically solution (b). To deal with complicated junctions, route indicators are also needed to distinguish between different turnout routes. The turnout indicator used in single light areas is essentially solution (a). Once again, route indicators may be needed to give additional route information. An alternative option is to use the route indicator only in conjunction with the main aspect (method (c)). British practice is based totally on method (c). The main signal is always used for all routes. Junction or route indications are provided to tell the driver which route is set. The problem does not end with deciding the form of the junction signal. Most turnout routes have a speed restriction associated with them. In most cases it is too late to inform the driver at the junction signal. He will require a prior warning of either the route to be taken or the speed to which he must reduce prior to the junction signal. Again, there are several options available. Repeat the junction or turnout information at one or more previous signals depending on the speed reduction required. In complex track layouts this would be totally impractical. Provision of route indicators at successive  junctions would need to display information for both the junction immediately ahead and for junctions beyond the next signals. Warn the driver of the need to reduce speed for a turnout at the signal before the junction signal but display detailed route information only at the junction signal. This solution is satisfactory for many situations but does not cope very well with junctions where different routes have widely different turnout speeds. The train willl either reduce speed so that it is unable to take full advantage of higher speed routes or the train taking the low speed turnout will receive insufficient warning. Restrict the aspects of previous signals without providing any junction information. The junction signal is held at stop or caution until the driver is close enough to see the route indication or turnout aspect. This solution is safe but has two disadvantages. The speed is often restricted more than necessary by the need for the driver to be within sighting distance before clearance of the signal. This type of control does not deal well with trains of widely different braking characteristics. SRA junction signalling is essentially to method (b) and British practice is mostly to method (c) although the need to give advance warning of a turnout has been  recognised at high speed junctions by  the provision of flashing single and double  yellow aspects approaching a signal set for a higher speed turnout. Many systems of signalling give a turnout indication (and an advance warning of  the turnout) regardless of whether or not the turnout is of lower speed than the main line. If there is little or no difference between the speeds of the different available routes, advance warning and/or approach control is not necessary for safety purposes. The provision of such indications and/or controls will only serve to delay traffic without any safety benefits. In practice the provision of junction signalling indications to the driver in most route signalling systems has been implemented in a very imprecise manner. A good  example is the use of a medium indication in advance of a turnout signal. In SRA practice this same indication is given for a signal set for a turnout or a signal at  caution for the main line route. The speed  requirements could be very different.  Consider a 100km/h main line.  For a 25km/h turnout immediately beyond the junction signal, the medium indication at the previous signal must be capable of enforcing a 75%  speed reduction. If the  signals are equally spaced, it is safe for a main line train to pass the junction signal at caution (for the straight route of course) at approximately 70% of full line speed.  In this case, the same signal indication is only required to impose a speed reduction of 30%. The driver has no means of knowing which of these conditions will apply at the next signal so we must  hope that he makes the correct decision! Before leaving the subject of signal aspects, we should examine the situations in which signals may not need to display all aspects. Approaching terminal stations (or terminal roads in through  stations)  the buffer stop is normally considered equivalent to a signal fixed at stop. The previous signal need only be capable of displaying a caution indication for this route. A signal with main routes leading up to it must always be a main. If it only has shunt routes ahead, it will however be permanently fixed at stop. In this situation it need display no other main aspects, only a shunt aspect. 2.3 Identification of Signal Types Having decided the appearance and available aspects of a signal, we may also  need to provide further information on or near the signal. For the benefit of drivers and other operating and engineering staff, it is important to identify each lineside signal. This is normally done by attaching an identification plate to the signal post. This plate may show some or all of the following information:- The signal box/control centre from which the signal is controlled or supervised The line on which the signal is located. A precise or approximate distance reference of the signal's position. A unique signal number. The provision of a unique signal number is particularly important when the signalman and driver are communicating with each other, to avoid messages being passed to the wrong person. However reliable we make our signalling equipment, the possibility of failure must still be considered and operating rules devised to deal with the situation. Because the driver has to obey different rules in the event of failure of the signal or its signal post telephone, different types of signal need distinguished from each other. 2.3.1 Automatic Signals An automatic signal must be identified as such because drivers are usually  authorised in certain circumstances to pass automatic signals at danger. This may be after a certain time has elapsed and/or if the driver is unable to contact the signalman. The train may proceed cautiously to the next stop signal. The driver must be prepared to stop  short  of  any obstruction. With SRA practice, automatic signals have offset lights or an additional plate with the letter "A" (white on black background). The BR automatic plate is a black  horizontal stripe on a rectangular white plate. 2.3.2 Semi-Automatic Signal At the time a driver is stopped at a semi-automatic signal, he may not know whether the signal is being operated as a controlled signal or an automatic signal. He must therefore be told which is the case or reminded to check. SRA semi-automatic signals have an internally illuminated letter "A" provided. This is lit when the signal is operating automatically (controlling signal box or ground frame unattended). When operating as a controlled signal, the "A" light will be extinguished. On BR, this type of signal has an identification plate similar to that of an automatic signal, but with the word "SEMI" above the black stripe. In this form the actual appearance of the plate is not the same as the symbol used on signalling plans. Operating rules allow the driver to pass a semi-automatic signal at danger in the same circumstances as for an automatic signal. He must also ensure that the ground frame is not in use and any ground frame points are correctly set. 2.3.3 Controlled Signals Generally, controlled signals need no further identification and will only carry a signal identification plate. 2.3.4 Distant Signals Because trains will never stop at a distant signal, there is no operational need for any special type of plate. The signal identity is normally shown for the information of staff. As SRA is now including a red aspect on distant signals (to cater for the possibility of a train standing between the distant and its associated stop signal), distant signals are identified to the driver as automatic signals. There may be a need to show other instructions to a driver in certain circumstances, in which case, a suitably worded notice can be attached to the signal structure if required. Such notices will only generally need to be read and acted upon while a train is standing at the signal. No particular sighting requirements are necessary during daylight but such notices must also be visible at night. External illumination or the use of suitable reflective material may be considered. 2.4 Construction This comprises the mechanical construction of the signal, its optical system, its electrical connections and its mounting at the lineside. The construction of any main signal must fulfil the following requirements:- It must be clearly visible by an approaching train between its initial sighting  point and a train standing at the signal. It must be vandal-resistant. It must be weatherproof and protected against corrosion in the intended working environment. Irregular indications due to incident light from other sources must be prevented. It must operate reliably with the minimum of maintenance attention Most modern signals use a separate light unit for each light to be displayed. The illumination of each lamp is controlled externally. Many older signals were of the "searchlight" type with a moving filter in front of the lamp. This required a relay type mechanism and arrangements for proving contacts in the restricted space of the signal head. The "multi-unit" type of signal is simpler to operate and  requires less maintenance. It also has a more efficient optical system. Main aspect signals are usually constructed of a metal casting, although some recent signal heads have been of fabricated construction. The signal head is sectioned off internally for each aspect. Light  must not be allowed to pass from one section to another. Modules of 1, 2 or 3 lights are bolted together to provide the required number of aspects. An access door is provided at the rear for maintenance purposes, with provision for padlocking . Each light unit contains its own doublet lens assembly - the inner lens is coloured for the required indication, while the outer is clear. On most signals, each lens has several stepped concentric sections which focus the light from the bulb into a beam. The optical system is designed to be very efficient, and produce a very narrow,  intense beam for good, long-range sighting, even in the brightest of daylight conditions. For long range signals, the divergence of the beam is usually about 5° but the outer lenses are also available with a wider angle of divergence, usually in the horizontal plane only, which is more appropriate for signals to be viewed at short range or across sharp curves. Note that it is not possible to use a mirror within the light unit due to the risk of reflecting external incident light back towards the driver of an approaching train. Less than half of the light output of the lamp is therefore available for use. For this reason modern lens assemblies have a very short focal length to maximise the amount of light output which can be used. Color Light Signal Head - Construction and Optical System Signal Head Circuit for a Single Light of a Multi-unit Head The outer lens often has a prism segment to direct a portion of the beam towards  the driver of a train standing at the signal. A hood is provided (usually above each unit) to prevent "phantom" aspects (caused by sunlight entering the unit and being internally reflected  by the lens). A black backboard is normally fitted to assist sighting. In tunnels, the requirements are much less stringent. Low ambient light will permit a lower light output (lower power lamp or smaller lens). Backboards are not necessary. The signals must often be smaller to fit the limited clearances when mounted on tunnel walls or at ground level. The signal lamps are mounted in plug-in holders, such that the main filament is aligned accurately with the focal point of the lens system. The standard bulb in use at present is the SL35, which is a tripole bayonet cap lamp with 2 filaments each rated at 12v, 24w. To prolong the life of the bulbs, they are slightly under-run. Opinion differs between different railway administrations as to the optimum voltage. There is obviously a compromise between light output and lamp life. Lamp voltage is usually set at about 10.8 - 11.7v. Signals are usually fed from the adjacent location at 110/120 volts and transformed down to the required lamp voltage in the signal head. A transformer with multiple  tappings provides the necessary voltage adjustment. It is essential for reliable operation that the lamp voltages are correctly adjusted. As well as providing for lamp voltage adjustment,  the  use  of  signal  head  transformers allows the cable from the location to be longer before voltage drop becomes a problem. In a.c. electrified areas, the higher supply voltage gives increased immunity against induced traction interference. A miniature, plug-in relay is provided for each aspect, which detects the current  being drawn by the main filament. If the main filament fails, the relay will de-energise and illuminate the auxilliary filament. These relays are also used to indicate the failure of the first filament to the signalman or maintenance technician. The transformer, terminal block and miniature  relay for each aspect are all contained within the signal head. A schematic of the wiring of a single light unit is shown. 2.5 Circuits The circuits for operating a signal will depend on the type of interlocking, the type of signal (controlled, automatic or semi-automatic) and the aspects to be displayed (single light, double light, 2, 3, or 4 aspect etc.) With control from a Solid State Interlocking, most information is processed at the central interlocking. Where control is from a relay interlocking (or similar) some of the control logic is provided at the location. Circuits for Solid State Interlocking controls will be dealt with in a later section. The circuits required to operate a signal controlled from a relay interlocking consist of the following:- Circuits in the interlocking which determine whether the signal may display a proceed aspect (and for which route). The form of these circuits will be determined by the control tables An operating circuit from the interlocking to the location adjacent to the signal. This conveys the command from the interlocking for the signal to clear to a proceed aspect. Aspect control circuits, indicating the state of the signal ahead, and therefore the aspect to be displayed. These may come via the interlocking or direct from the signal ahead. Generally, one relay will be needed for each possible aspect to be displayed. Location circuits which determine the main signal aspect and any route indication lamps to be illuminated. Circuits via tail cables from the location to the signal and route indicator lamps. Indication circuits from the location to the interlocking to show the actual condition of the signal. Aspect control circuits (as (c) above) to signals in rear. Filament failure indication circuits Some means of warning the driver and/or restricting the approach of a train if the signal is not displaying a light If a trainstop is provided, additional circuits will be necessary to ensure that the signal and trainstop operate together in a coordinated manner.  This is covered  in  more detail in part 8 of these notes. Similarly, if an automatic warning system or automatic train protection equipment are provided, an interface with the signal circuits will be necessary. A simple example of single light colour light signal circuits is shown in the following diagrams. A separate relay is energised for each aspect. The HR controls the yellow (caution)  aspect, the HDR controls the pulsating yellow (medium) aspect, where provided,  and the DR controls the green (clear) aspect. The HDR requires the HR energised and the DR requires both HR and HDR up. This is to ensure a correct aspect  sequence between signals and will fail safe in the event of disconnection of any of the circuits. The HDFR switches a resistance in the yellow light circuit to provide the pulsating light. Again this is fail safe because a failure of the flasher will result in a steady light. Three Aspect Automatic Signalling - Typical Circuits Four Aspect Signal Circuits (Single Light) 3. SHUNTING SIGNALS 3.1 Aspects In addition to main signal aspects, other aspects need to be displayed for shunting moves. These are moves in which the line ahead is not necessarily clear. The requirements are different to main signals. Firstly, the signal needs to display only one proceed aspect. All shunting moves proceed at relatively low speed and  the aspect of the next signal (if any) is not relevant. The aspect must be sufficiently different from that of a main signal so that its meaning is clear. On most railways, subsidiary shunt signals (i.e. those mounted on the same post as a main signal) need not display a stop aspect as the main signal is already provided with one or more red lights. Three types of shunt signal are in common use. A miniature version of a full size main running signal. The stop aspect is almost invariably red. The proceed aspect chosen by different railways has usually been yellow although white and green have been used. Retention of some type of semaphore signal (e.g. a motor or solenoid operated disc). These will require internal or external illumination at night. A "position light" signal. The aspects consist of two or more lights.  The orientation of the lights is significant as well as the colour. SRA has adopted the first of these options. Although some shunt signals are termed position light, the aspects displayed are the same as for the miniature or dwarf colour light signal, yellow for proceed and two red lights for stop. Only the orientation of the two red lights is different. British Rail uses a position-light shunt signal with 3 lamps mounted in a triangle. It has 4 basic forms:- Ground Shunt : the bottom-left-hand lens is red, the other two are white. Subsidiary : used in conjunction with a main aspect signal, which provides the stop (red) indication (which remains illuminated when the subsidiary signal  is in use). The subsidiary is only illuminated for a proceed aspect, so the red  lens is blanked off. In addition to shunting moves, subsidiary signals are also used for "call-on" moves to join two trains together, usually in platforms. Limit of Shunt : this signal is a special version of the ground-shunt, which never shows a proceed aspect. Accordingly, the upper lens is blanked off, and both lower lenses are red. It is a direct replacement for the former limit of shunt  board  and  acts as the limiting signal for a wrong line shunting move. Yellow Shunt : this is a special version of the ground-shunt. It is now very rarely used. To permit shunting without repeated route setting by the signalman, it may be passed when "on" provided the points are set for a specific route. The "off" aspect will apply to all other routes. It has a yellow lens in place of the usual red lens. 3.3 Construction The construction of a position-light signal is much simpler than that of a main signal, as there is no requirement for long-range sighting. On the BR position light, each lamp has two lenses. For white lights, the outer lens is lunar white and the inner lens clear; for red or yellow lights, the outer lens is clear and the inner lens coloured. Lamps are fed direct from 110v  without needing a transformer.  Single filament  bulbs are used, with no auxilliary filament. Position Light Signal Construction 3.4 Circuits   Assuming that no route indicator is necessary, the controlling circuit for a shunt signal is much simpler than that of a main signal. Only one controlling relay is needed. The relay must be energised to operate the proceed aspect. No information on the aspect of signals ahead is included. To further simplify the circuits, the following features of main signal circuits may be omitted:- Twin filament lamps. Many shunt signal lamps are single filament (or only one filament is used). Lamps are often fed direct from the mains supply (110/120 volts) without a transformer. If there is no filament changeover facility, filament failure indication circuits will not be provided. Lamp proving. The dangers associated with a lamp failure are much less than for a main signal due to the low speed. The risk is further reduced if the stop aspect displays more than one light. BR provides lamp proving for "limit of shunt" signals only. Aspect control circuits to/from adjacent signals. 4. ROUTE AND JUNCTION INDICATORS 4.1 Types The following summarises the available types of route and junction indicators in common use:- a.Main signals Route indicator displaying a character and working in conjunction with the main signal aspect Multi lamp ("theatre" type) Fibre-optic Directional route indicator working in conjunction with main signal aspect. BR position light junction indicator Directional route indicator working independently  (main signal aspect remains at stop) SRA turnout indicator b.Shunt signals Route indicator displaying a character and working in conjunction with the shunt signal proceed aspect Back-lit stencil Multi-lamp Fibre Optic Each type will be discussed briefly. An essential requirement is that any route indication working in conjunction with the primary signal aspect must have a similar range and intensity so that both are visible together. 4.2 Multi-lamp Route Indicators The multi-lamp (or "theatre") route indicator (MLRI) is always used  in conjunction  with a main signals, never alone. It has a reasonable medium range visibility but not as good as a colour light signal due to the need to display a character rather than just a colour. On BR, it may only be used when the speed over the indicated route is 40 mph (64km/h) or less. MLRIs are normally used at the approach to terminal or bay platforms. SRA also uses miniature MLRIs in conjunction with dwarf colour light shunt signals. There are two slightly different applications for main signals:- 4.2.1 SRA Double light Signalling and BR Signals With a High Speed Route No route indication is displayed for the main route. All other routes will display a distinct letter or number. This will cause no confusion with SRA signalling as the straight route will be associated with a green light in the main head and all other routes with the yellow light. On BR, as the signal could show a green light with a low speed turnout set, it is essential to prove the route indicator alight before the signal is allowed to clear. 4.2.2 SRA Single Light Signalling and BR Signals for Low Speeds Where all routes are of a similar speed there is no need to prove the route indicator alight before clearing the main signal. SRA single light signals use route indicators where there are several routes of a similar speed or there are two or more left hand or right hand turnouts. In the second situation a turnout indicator alone is insufficient. The turnout indicator will display the three yellow lights to the left or right as appropriate. The route indicator will show which of the left hand or right hand turnouts is set. 4.2.3 Circuit Operation Each route indication consists of a number or letter, formed by illuminating the necessary lamps in a matrix (usually 7x7 lights). The indicator has a lunar-white, glass front cover. There will normally be a controlling relay for each indication. The lamps will be wired in parallel groups as far as possible. The operation of the controlling relay will complete the circuit to the correct lamps or groups of lamps to display the required letter or number. Each indicator therefore has to be individually wired to display the correct combination of letters and/or numbers. Lamp proving may be difficult to arrange precisely because different characters will require a significantly different number of lamps. For example, "1" will generally require only 8 lamps while "8" would require at least 21 lamps illuminated. A lamp proving relay in series with the route indicator would have to be adjusted to operate with no more than 8 lamps alight. This could allow the signal to clear for other routes with less than half the lamps illuminated. Multi-lamp Route Indicator 4.3 Fibre Optic·Route Indicator The problems of lamp proving have been solved with the introduction of  the fibre  optic route indicator. In appearance it is similar to the multi-lamp indicator but each indication is illuminated by its own single lamp. BR uses a 12v 55w quartz halogen  lamp similar to a car headlight. The light is conveyed to the correct positions on the matrix by a number of glass fibres. The arrangement of fibres from each lamp to the face of the route indicator will of course be unique to each route indicator. Fibre optic route indicators are available in full size and miniature form according  to whether long or short range sighting is required. The miniature type is suitable for use with shunt signals. Fibre Optic Route Indicator 4.4 Position Light Junction Indicator The position-light junction indicator (PLJI) is used on BR in conjunction with main signals for indicating which route has been set through a junction.  Each indication  consists of 5 white lights in a row. Six possible indications are available, 3 each for  left-hand  and right-hand divergences. Normally no indication is given for the straight route. The latest PLJIs use an efficient optical system giving high light output and good long range sighting. SL35 12v lamps (the same as used in main aspect signal heads) are used. 110v/12v transformers are mounted within the junction indicator.  Note that only the main filaments are used on each lamp, filament failure indications are not provided. If only one diverging route is indicated, all 5 lamps are simply wired in parallel to the transformer 12 volt output. Where two or more diverging routes are indicated, then one lamp (the pivot) is common to all indications.  A separate circuit is therefore required for the pivot, operated via contacts of all the junction indicator control relays in parallel. As the PLJI does not indicate the high-speed route, the failure of all lamps on the PLJI could mislead the driver by appearing to be a high speed indication. To avoid this risk, the clearance of the main signal must therefore prove the PLJI has lit. A series lamp proving relay is provided in the operating circuit. At least 3 bulbs must be lit before the main signal clears. It is customary to always lamp-prove PLJIs - even where there is no straight route  or where they are used with subsidiary signals for call-on moves. No other indication of lamp failure is provided. It is therefore necessary to check periodically for failed lamps. Position Light Junction Indicator 4.5 Turnout Indicator This has a number of similarities to the BR junction indicator. It displays a row of lights inclined in the direction of the turnout. The differences are:- The signal works independently - the main aspect remains at red. The signal displays yellow lights. Only three lights are displayed. Only two possible displays are available, 45° to the left or right. Multiple turnouts  to the same side of the main route must be distinguished by the use of a multi lamp indicator in conjunction with the turnout indicator. It can display two indications, either a steady or a pulsating light. As the main signal remains at red, this allows the turnout indicator to advise the driver of  the state of the next signal ahead. Turnout Indicator Construction Turnout Indicator Circuits 4.6. Stencil Route Indicator In construction and operation, these are the simplest of all route indicators. They are also the cheapest to manufacture. They consist of one or more lamps mounted in a box behind a stencil. When the lamps are illuminated, light shines through the cut-out of  the stencil on to a ground glass screen which diffuses the light sufficiently to display the characters clearly. Due to their short range visibility, stencil indicators are normally only used in conjunction with shunt signals. As such, they are not normally lamp-proved. Where a stencil indicator is provided, it is usual to provide a route indication for each available route. Stencil Route Indicator Construction and Typical Circuit 5. REPEATER AND INDICATOR SIGNALS Due to inadequate sighting distance or the need for a signal to be seen by operating staff other than the driver, additional signals may be needed to repeat the aspect of certain signals. The location of such signals must be chosen with care. They must be visible to an approaching train, platform staff, guards or shunters without misleading the drivers of trains on adjacent lines. 5.1 Banner Repeater These signals are used on BR principally for repeating main signals, where the sighting distance of the main signal is too short. It consists of a black band on a white background. The black band horizontal indicates that the main signal ahead is at red (stop), and inclined at 45° to horizontal that the signal ahead is cleared to at least a caution. It may be possible to avoid installing banner signals by careful positioning of main signals. Originally, banner signals were all operated electro-mechanically. Inside the signal, the black metal band is pulled to the clear position by an electro-magnet. The signal is internally illuminated so that it may be seen at night. This type of banner signal is generally less reliable than a colour light signal and requires "on" and "off' proving contacts to ensure that it is operating correctly. These will be included in the controls of the signal in the rear and the signalman's panel indications. Proof of signal "on" in the approach lock release circuit will also require the banner signal normal. If there is a separate track circuit between the banner and its main signal, the banner is not allowed to clear if there is a train ahead of the banner. This provides additional protection when working trains under failure conditions. 5.2 Colour-light Indicator and Co-acting Signals This type of signal is also used in similar situations to the banner signal. It  is  usually located close to the position of the main signal repeated and repeats all the main signal's aspects. A physically smaller signal may be used. Common  situations  for use of these signals are:- Where the view of the signal is obstructed by station buildings, bridges, tunnels or trains standing on adjacent lines Where the driver of a train standing at the signal would have difficulty seeing it, possibly where the main signal is mounted high above the track or on the opposite side of the train to the driving position. 5.3 Platform Indicators Guards' indicators or "off' indicators are often provided where train crew and/or  platform staff cannot see the platform starting signal. It may assist them in controlling passengers, operation of automatic doors and giving  instructions  for  the train to start  (a driver should not be given an instruction to start his train if the platform starting signal is at danger). "Right away" indicators are positioned within view of the driver of a stationary train  to enable platform staff to give an instruction to start. BR employs stencil indicators for both types. SRA has a special lunar white signal for the guard's indicator. This must be located in an elevated position on the platform so that the guard can see it from his normal position(s) on the train. Allowance  must be made for different lengths and/or types of train where necessary. A double sided signal is normally used. 5.4. Driver's Level Crossing Indicator British level crossing requirements include continuous monitoring of the correct  operation of all level crossings. A periodical test is not considered adequate. Many automatic level crossings are not monitored by a signal box but depend on the driver to check that the crossing is operating correctly. A white signal having an  optical system similar to a main signal is located at the crossing. It is normally unlit. When it displays a flashing indication, it proves to the driver that the crossing is closed to road traffic by the correct operation of road signals. The driver must  check for himself that the crossing is clear of any obstruction. Line speed must be restricted so that sighting distance of the crossing and the signal is greater than emergency braking distance. Warning boards are also provided on the approach to the crossing. 6. FIXED DISTANT SIGNALS To simplify the operation of the signalling and economise on the provision of control equipment and power supplies, a distant signal may be replaced by some form of fixed indicator or board. It avoids the provision and maintenance of a distant signal at locations where all trains stop (or reduce to a very low speed). The SRA version is the "landmark" · signal. The BR version is a horizontal yellow band in the same shape as a semaphore distant arm on a white background. Both types are reflective. This ensures that it is visible at night provided all trains are equipped with headlights. On some urban rapid transit railways a marker may be provided at braking distance from stop signals. This is not strictly a fixed distant but is to ensure that the driver commences braking at the correct point if the associated stop signal is at danger. Braking distances are generally short and distant signals are generally not necessary. The stop signal must be visible from the position of the marker board. If not, a distant signal should be provided. 7. EMERGENCY SIGNALS On many urban railways, particularly where trains are operated by one person only, a facility may be provided for a stop signal to be displayed to the driver if an emergency arises on departure from a station. These signals take various forms according to the preferences of each railway administration. They are usually operated by passengers or staff from one or more plungers on the platform. 8. TRAINSTOPS A trainstop is the simplest form of automatic train protection. In conjunction with an overlap of adequate length, it can ensure that a train which passes a signal at stop will be brought to a stand before it reaches another train or a conflicting route already set. The theory of positioning trainstops has already been covered in "Signalling a Layout". This section will concentrate on how they are used in conjunction with signals to protect a train which has overrun the limits of the route set. The use of trainstops is most effective on urban rapid transit systems where speeds are relatively low and the trains have a short braking distance. On many such railways distant signals are not provided, the driver being able to see the stop signal at the point where he must start braking. On some railways marker boards are provided to indicate the start of braking for the stop signal. 8.1 Positioning For trainstops to be effective, a complete line or area of railway must be  fitted  and  the majority of trains working over that line must be equipped to operate with the trainstop equipment. Trainstops are required at every main signal capable of  displaying a stop indication to the driver. Trainstops may also be used in certain other locations:- Approaching terminal platforms to ensure the speed of a train entering  the platform is controlled to a safe maximum. Approaching signals with a very short overlap beyond. The lowering of the trainstop is controlled by the occupation of an approach track circuit. The approach of a train at excessive speed will result in the brakes being applied before the train reaches the signal at which it is to stop. This ensures that the train does not overrun the end of the overlap. Although electro-magnetic trainstop equipment is now available which will operate  without any physical contact between train and track mounted equipment, this section of the notes will concentrate on the conventional  trainstop which acts directly on a trip lever on the train to exhaust the air from the train's braking system. To operate the train braking system effectively, all trainstops must be mounted in the same position relative to the train. Standard measurements must be used for the height of the trainstop arm above or below rail level and its horizontal distance from the running rail. Tolerances must take account of the effect of curvature and, most importantly, all trainstops must be mounted on the same side of the line. SRA  provides trainstops on the left hand side of the line. British Rail and London Underground position all trainstops on the right hand side. 8.2 Mechanical Operation The arm of the trainstop must be sufficiently strong to withstand the impact of the  train's trip lever at the maximum train speed. Although most instances of tripping  are likely to occur at low speed, the occasional higher speed operation should not damage the equipment. The trainstop arm is therefore of substantial dimensions. Even with a sufficiently robust arm, the risk of the arm being damaged or broken is still significant. The absence of the arm is a wrong-side failure. It must therefore be protected by proving that the arm is in the correct position in other signal controls. An essential part of the trainstop is its proving mechanism.  A linkage to the arm, as near to the outer end as possible, drives a circuit controller with a number of  contacts to indicate the trainstop normal (raised) or reverse (lowered) to various tolerances. The absence of the arm will result in neither normal nor reverse contacts being made. The arm itself must be lowered and maintained down by the continuous energisation of the trainstop control circuit. This normally acts against a spring and/or a counterbalance weight so that a disconnection of the control circuit will result in the trainstop returning immediately to normal. Operation can be by electric motor, air pressure or a hydraulic system. Electric trainstops are normally driven down  by an induction motor. To avoid  damage to the motor when the trainstop is continuously held down for long periods (e.g. at automatic signals) a lower power holding mechanism must be provided. On  some types of  trainstop, the motor has two sets of windings,  one to drive the arm  down and a lower power winding to hold the trainstop reverse. Cutoff contacts are provided in the circuit to disconnect the main windings. Alternatively, a locking mechanism may be operated by a solenoid. Power disconnection will de-energise the solenoid, releasing the lock and allowing the arm to return to normal. Electro-pneumatic trainstops are controlled by a valve which, when energised,  allows air to enter a cylinder, driving a piston which drives the trainstop down. De-energising the valve exhausts the cylinder and a return spring raises the trainstop to the normal position. Hydraulic trainstops are operated by a combination of an electrically driven pump and a solenoid valve. Aplication of power to the trainstop pumps fluid into a cylinder which drives  the arm down. The solenoid closes the fluid return path to the cylinder.  When the arm is fully lowered, the pump motor is switched off and the arm is hydraulically locked down. Disconnection of power releases the solenoid valve and spring pressure raises the arm and returns fluid to the main reservoir. 8.3 Circuits It is impossible to deal with trainstop circuits in isolation.  For correct and safe operation they must be studied in conjunction with the signal control circuits. The example shows typical circuits for a section of 4 aspect automatic signalling as used  in the Sydney metropolitan area. The HR, as with any other signal, operates when all signal controls   are present.  Included in this circuit is the VCSR for the signal ahead. This relay proves that the trainstop of the signal ahead has returned to normal after the passage of the previous train. Unlike other signals, the HR alone is not sufficient for the signal to show a proceed aspect. Operation of the HR will complete the circuit to the VR which will then drive the trainstop down. The reverse proving relay (VRR) will energise to complete the circuit to the top green light and disconnect the top red light. The purpose  of the control  is to stop the train at the signal if the trainstop fails to lower, rather than tripping the train at speed. Both HR and VRR are required up in the control of the medium and clear aspects of the signal in rear. When the signal returns to danger as the train passes it, the VR will de-energise and  the trainstop will raise. To prove this has happened, the VCSR will only energise when the first track past the signal clears and the trainstop is normal. Failure of the VCSR to pick will prevent the previous signal from clearing. When the signal is ready to clear, the HR contact in the VCSR circuit will hold the VCSR up, maintaining the aspect circuits to the rear. This contact also enables traffic to continue running if a trainstop fails by allowing a train to approach the failed trainstop if the signal has been cleared. Depending on the arrangement of the tripcock equipment on the trains, trainstops may need to be lowered for wrong direction moves. A typical trainstop suppression circuit is shown. Of particular importance is the proving of the suppresion relays normal before a train is allowed up to a trainstop which has been suppressed. 4 Aspect Signalling with Trainstops - Typical Circuits Signail Lighting Circuits Trainstop Suppression Circuits

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Deepu Dharmarajan -
Posted 97 days Ago

SIGNALLING A LAYOUT

CONTENTS 1. Introduction 2. Headway 3. Positioning of Running Signals 4. Types of Signal 5. Points and Crossings 6. Track Circuits 7. Identification of Signals, Points & Track Circuits 8. Examples 1. INTRODUCTION One of the first steps in any signalling project is to determine the method of train working. Having decided this, it is then necessary to decide the position and spacing of signals. This section will assume throughout that colour light signalling to track circuit block principles will be provided on all main lines. Although other methods of working may well be more appropriate, particularly for lightly used single lines, these will be covered later in the course. It is useful at an early stage to determine whether 2, 3 or 4 aspect signalling will be required. This will be governed by the required line capacity, which in turn will be determined by the timetable to be operated. Having this information and an approximate signal spacing, we can then proceed to position the signals on a scale plan of the track layout. Their position relative to stations, junctions etc. will be decided largely by operating requirements. The most economical arrangement that meets all operating requirements is the one that should be adopted. In order to produce a safe and economical signalling scheme, the designer must use his knowledge of signalling principles and be provided with all necessary details of the train service pattern required, the track layout, gradient profiles, line speeds and train characteristics. If this information is not immediately available, it must be requested from the appropriate authority. Sometimes operating requirements conflict with each other and with safety standards — the engineer must then use his experience to reach a satisfactory compromise whilst maintaining the safety standard 2. HEADWAY The headway of a line is the closest spacing between two following trains, so that the second train can safely maintain the same speeds as the first. This usually means that the second train is sufficiently far behind the first that its driver does not see an unduly restrictive signal aspect. Headways can be expressed in terms of distance but more usefully as a time (e.g. 2 1/2 minutes between following non-stop trains). It can also be converted to a line capacity (trains per hour). Care must be taken when using a "trains per hour" figure if the trains are not evenly spaced in the timetable. The signalling must be able to handle the minimum headway, not the average. Headway will depend on a number of factors:— D = Service Braking Distance d = Distance between STOP signals S = Sighting Distance (usually 200 yds/metres or distance travelled in 10 seconds) O = Overlap Length L = Train Length (less than 100 yds/metres for a short suburban train but possibly over 1km for a heavy freight train) V = Line Speed (or actual train speed if lower) a = Braking rate Where any of these factors are not given to you, you should always state your assumptions. In practical situations, it is vital to obtain accurate information regarding the braking performance of trains. It is also vital to standardise your units of distance and time. If you work in imperial, yards and seconds are most useful; in metric, metres and seconds would be most appropriate. Whichever you decide, you must use the same set of units consistently throughout to avoid confusion and error. 2.1 Service Braking Distance This is the distance in which a train can stop without causing undue passenger discomfort. It will depend on the line speed, gradient, and type of train. It is usually significantly greater than the emergency braking distance. Theoretically, the Service Braking Distance can be calculated using the line speed and braking rate                                                                     (Line Speed) 2                V 2 Service braking distance (D)                   --------------------------                                                                     2 x (Braking Rate)       2a This is derived from the 3rd Law of Motion. This calculation will depend upon the braking characteristics of the type(s) of train using the line and must take into account the worst case combination of train speed and braking rate. If this calculation is to be performed frequently, it is useful to show the service braking distances for different combinations of speed and gradient in tabular or graphical form. Gradient should always be taken into account. A falling gradient will increase braking distance, a rising gradient will reduce it. As gradients are rarely uniform between signals, we need to calculate an average gradient using the formula: where G is the average gradient  D is the total distance g and d are the individual gradients & distances. For a gradient of 1 in 100, G = 100. If the gradient is expressed as a percentage, G is the reciprocal of the percentage gradient. Falling gradients taken as negative, rising gradients as positive. 2.2 2 Aspect Signalling 2 aspect signalling will generally be adequate on lines where traffic density is low. The required length of block section is much greater than braking distance. Only two types of signal are used, a stop signal showing stop and clear only and a distant signal showing caution or clear. Each stop signal will have its associated distant signal. As 2 aspect signalling will mainly be found outside the suburban area, the example shows single light signals. The distance (d) between stop signals is variable according to the geography of the line, positions of stations, loops etc. The headway distance can be calculated as: H = D + d + S + O + L giving a headway time: Note that the headway time for the line is that of the longest section and cannot be averaged. To obtain the greatest signal spacing to achieve a specified headway, we transpose the equation to give: d = (V x T) - ( D + S + O + L) 2.3  3 Aspect Signalling With 2 aspect signalling, as the required headway reduces, each stop signal will become closer to the distant signal ahead. it is therefore more economic to put both signals on the same post. This then becomes 3 aspect signalling. Each signal can display either stop, caution or clear. The distance (d) between signals must never be less than braking distance (D), but to ensure that the driver does not forget that he has passed a distant at caution, d should not be excessively greater than the service braking distance. The current SRA recommendation is for signal spacing to be no greater than three times braking distance. BR has adopted a maximum of 50% (i.e. 1.5D) although this is often exceeded at low speeds. The headway distance is given by:- H = 2d + S + O + L So the best possible headway, when the signals are as close as possible (exactly braking distance), is: H = 2D + S + O + L The headway with signals spaced 50% over service braking distance is: H = 3D + S + O + L The headway with signals spaced at three times braking distance is: H = 6D + S + O + L 2.4  4 Aspect Signalling Where signals are closer together than braking distance, a preliminary caution or medium aspect is needed to give trains sufficient warning of a signal at danger. This medium aspect must not be less than braking distance (D) from the stop aspect, so the distance (d) between successive signals must on average be no less than 0.5D. The headway distance is given by:- H = 3d + S + O + L       where d > 0.5 D  So the best possible headway with 4 aspect signalling is given by:- H = 1.5 D + S + O + L In practice, the geographical constraints of the track layout will probably prevent regular spacing of signals at 0.5D. If the total length of two consecutive signal sections is less than braking distance, an additional medium aspect will be required at the previous signal. In other words, the first warning of a signal at stop must be greater than braking distance away. If more than two warnings are required, the medium aspect is repeated, not the caution. Signals should however be positioned so that this situation is as far as possible avoided. 2.5. Application of Low Speed Signals and Conditional Caution Aspects In normal use, the addition of a low speed signal provides the driver with a fifth aspect. It is important to realise that this does not have any effect on the headway of through or non-stopping trains running at their normal speed. In this situation, the engineer will arrange the signals so that each driver should, under normal conditions, see only clear aspects. The preceding headway calculations apply regardless of whether low speed signals are provided or not. A low speed signal tells the driver that he has little or no margin for error beyond the next signal and should control the speed of his train accordingly. The benefit of low speed signals is in allowing a second train to close up behind a stationary or slow moving train by reducing the length of the overlap, provided the speed of the second train has been sufficiently reduced. The same effect can be achieved by delaying the clearance of the caution aspect. This is now preferred provided an overlap of the order of 100 metres can be achieved. The clearance of the signal should be delayed to give a passing speed of approximately 35km/h. Low speed signals should only be used where the reduced overlap is very short (less than 50 metres) and/or there are fouling moves within 100 metres of the stop signal. 2.5.1 Station Stops With an overlap of 500 metres, a train stopped at a station will have at least 500 metres of clear track behind it. A following train will stop at the first signal outside this distance. By the addition of a low speed signal or a conditionally cleared caution, the overlap distance can be reduced and the second train can approach closer to the station. When the first train leaves the station, the second train can enter the platform earlier, thus giving a better headway for stopping trains. A conditionally cleared caution aspect will normally be used unless the overlap is less than 50 metres. 2.5.2 Approaching Junctions Trains awaiting the clearance of another movement across a junction can approach closer to the junction while keeping the overlap clear of other routes across the junction. A low speed aspect will normally be used in this situation. 2.5.3 Recovery from Delays A line which is operating at or near its maximum capacity will be susceptible to disruption from even minor train delays (e.g. extended station stops at busy times). Low speed signals and or conditionally cleared caution aspects can allow trains to keep moving, even if only slowly, to improve recovery from the delay. The total length of a queue of trains will be less and the area over which the delay has an impact will be reduced. 2.6 Summary For 2 aspect signalling, the headway distance is:- H = D + d + [S + O + L] For 3 aspect signalling, the headway distance is:- H = 2D + [S + O +  L]   (minimum) where signals are spaced at braking distance H = 2d + [S + O+ L] (general case) for an actual signal spacing of d For 4 aspect signalling, the headway distance is:- H = 1.5 D + [S + O + L] (minimum) where signals are spaced at braking distance H = 3d + [S + O +  L]  (general case) for an actual signal spacing of d Note the factor [S + O + L] is common to all equations. Headway time is then calculated as:        H T = -----         V 2.7 Determining Signal Type and Spacing Because cost is generally proportional to the number of signals, the arrangement of signalling which needs the smallest number of signals is usually the most economic. It must, however, meet the headway requirements of the operators. For non-stop headways it is likely that the same type of signalling should be provided throughout. Otherwise there will be large variations in the headway. Remember that the headway of the line is limited by the signal section which individually has the greatest headway. This section will briefly describe a technique for determining the optimum signalling for a line. There may need to be localised variations (e.g. a 2 aspect signalled line may need 3 aspect signals in the vicinity of a station or a 3-aspect line may need to change to 4 aspect through a complex junction area). These variations will depend on the requirements for positioning individual signals and can be dealt with after the general rules have been determined. Firstly, determine braking distance, train length and overlap length required. Each must be the worst case. Knowing the required minimum headway, use the H = 2D + S + O + L equation to determine the best possible headway for 3 aspect. Compare the results with the required headway to check whether "best case" 3 aspect signalling is adequate. There should be a margin of 25–30% between the theoretical headway and that required by the timetable to allow for some flexibility to cope with delays. 2.7.1 If the Headway is Worse than Required 3 aspect will not be adequate and 4 aspect must be used. Recalculate for 4 aspect to confirm that this does meet the headway requirement. T = (1.5D + S + O + L) / V If the non-stop headway requires 4 aspect signalling, it is likely that station stops will cause further problems. Signal spacing near stations should be kept to a minimum and low speed signals or conditionally cleared cautions with reduced overlaps may also be required. , 2.7.2 If the Headway is Much Better Much better generally means a headway time of 30% or less than that required by the timetable. If this is the case 2 aspect will generally be adequate. Calculate the greatest signal spacing that will achieve the headway with 2 aspect signalling. d=(V x T) — (D + S + O + L) Remember that in this distance d there will be two signals, a stop signal and a distant signal. Then compare this with the maximum permissible signal spacing for 3 aspect. In the absence of any firm rules, a judgement must be made on the amount of excess braking which is acceptable. SRA recommends that signal spacing is no more than three times braking distance while BR signalling principles specify no more than 1.5 times braking distance. If the two calculations produce a similar total number of signals (i.e. d for 2 aspect is approximately twice the value of d for 3 aspect) a 3 aspect system will be the better choice. The cost of the signals will be similar and the operator may as well benefit from the improved headway provided by 3 aspect. 2.7.3 If the Headway is Slightly Better It is probable that 3 aspect is the correct choice. Check that there is sufficient margin between the required and theoretical headway. 2.7.4 Signal Spacing Having evaluated that the chosen arrangement of signalling will provide the required headway, the relevant equation should be transposed to calculate the greatest possible signal spacing that can be allowed with the specified headway: eg. for 3 aspect signalling: V x T = H           = 2d + S + O + L therefore  2d     = (V x T) - (S + O + L) from which the post to post spacing (d) can be calculated Remember, there may be a constraint on the maximum signal spacing. The value of d should not exceed this. Geographical constraints may also require signals to be closer together than braking distance, in which case the 4th (medium) aspect is used where required. It does not need to be used throughout unless for headway [puposes]. 2.8. Example Information given:- Max. Line Speed...... 90 km/h Gradients ..........  Level Train Length.............. 250 metres Headway Required..... 2 1/2 mins. (non-stop) Before we start, we need the Service Braking Distance, either by calculation or from tables/curves (where available). We will assume that D = 625 metres. Note : S assumed to be 200 metres. O assumed to be 500 metres (although overlaps may need to be more accurately calculated if trainstops used) V = 90 km/h = 25m/s First, check 3 aspect signalling:- H = (2D + S + O + L)        = (1250 + 200 + 500 + 250)        = 2200 metres so T = H/V = 88 seconds. This is much less than the 150 seconds (21/2 mins) specified. We will therefore consider the alternative of 2 aspect signalling. We cannot calculate a theoretical headway for 2 aspect signalling as the signal spacing is not fixed. Instead, we calculate the greatest 2 aspect signal spacing to give us the 150 second headway specified : d = (V x T) - (D + S + O + L) = (25 x 150) - (625 + 200 + 500 + 250) = 3750 — 1575 metres = 2175 metres Hence 2 aspect signalling, with the stop signals no more than 2175 metres apart, would give the 2 1/2 min. headway required. However, each stop signal also requires a distant signal. Two signals are therefore required every 2175 metres. 3 aspect signalling with signals every 1088 metres would require no more signals but would give a better headway of: H = 2d + (S + O = L) = 2175 + (200 + 500 + 250) = 3125 metres So T = H/V = 3125/25 = 125 seconds In fact, the signal spacing could be extended further within the headway requirement of 150 seconds. This would give a better headway with fewer signals than 2 aspect. This demonstrates that 2 aspect is generally worth considering only for very long headways. We could now calculate the maximum possible 3 aspect signal spacing allowed by the headway : V x T = H = 2d + S + O + L therefore 2d = (V  x  T) — (S + O + L) = (25 x 150) — (200 + 500 + 250) = 3750 — 950 = 2800 metres d = 1400m As this is over twice braking distance, it should be confirmed that this signal spacing is operationally acceptable 3.0 POSITIONING OF RUNNING SIGNALS If we are starting with a blank track layout, we need a logical method of setting out the signals in order to produce a signalling plan. As running signals must fulfil the needs of both headway and braking distance, it is usual to position running signals first. Shunting and subsidiary signals are dealt with after the running signals have been placed. 3.1 Headway Constraints First plot the positions of the running signals on the principal running lines. Then proceed to lower priority lines in order of importance. It is often difficult to decide where to start. If a station exists on the layout it is usual to start by plotting platform starting signals, and then continue by plotting the signals in rear and in advance to be within the tolerance of minimum separation (for braking distance) and maximum separation (for headway) for the type of signalling to be employed. It is also desirable to position signals close to facing points at junctions so that the driver does not have a long distance to travel to the turnout. This will minimise delay to traffic and reduce the possibility of signals being misread. Service braking distance must be provided from the first cautionary aspect to the signal at danger in every case. The maximum distance from the first caution to the red is set by the headway requirement. Ensure that any large excess over service braking distance is within acceptable limits. 3.2 Other Constraints In addition to headways, other constraints must be borne in mind These include junctions, further stations, tunnels, viaducts and level crossings. There may be many places where it is required to stop a train for operating reasons. The engineer will have very little choice in the position of these signals. Conversely there are also many places where it is highly undesirable to stop a train and signals must be positioned to avoid these. Other factors to consider are the visibility of the signal to the driver, the practicality of installing a signal at a particular site and ease of access for maintenance. Signal A has been placed at a distance greater than service braking distance from B because it would have otherwise stood on the points 201. If a line is generally signalled with 2 or 3 aspect signals and it seems necessary to have two signals spaced closer together than service braking distance, remember that the next signal in rear must be capable of displaying a medium aspect to give an earlier warning of the need to stop. If a suitable signal does not already exist, an additional signal will have to be introduced. A signal should be positioned so that a train stopped at that signal does not: a. Stand in a tunnel (wholly or partially). This would not apply to underground passenger railways where special provision has been made in the design of the trains and/or tunnels to ensure the safe evacuation of passengers in an emergency. b. Stand on a viaduct, unless special provision has been made for safe evacuation of passengers and/or access by emergency services. c. Foul a junction. d. Stand partially at a platform (unless the passenger doors can be kept closed by the train crew). There are occasions when, due to local circumstances, these requirements cannot be wholly met but every effort should be made to comply. Additional considerations will apply on electrified lines to ensure that trains are not brought to a stand in neutral sections or gaps in the conductor rail. If possible, heavy freight trains should not be stopped on steep falling or rising gradients, especially in combination with sharp curves. 3.3 Examples of Standage Constraints a)  Ensure maximum length train can stand in platform. b) Ensure that maximum length train can stand clear of junction fouling point to allow other movements to pass behind it. c) Signals on adjacent running lines should as far as possible be placed opposite each other. This minimises the chance of drivers reading the wrong signal. It also simplifies the design and installation of power supply and location equipment. 4.0 TYPES OF SIGNAL Running signals may be divided into three general groups according to the form of control exercised by the signalman. 4.1 Automatic Signals Automatic signals are designed to operate only according to the presence of trains on the track circuits ahead. The signalman does not have to set the route for each train. Usually he will not have any facility to set routes but may in certain circumstances be provided with a switch or button to replace the signal to danger in emergency. A signal may be shown as automatic if all the following conditions can be met:- a) No points in the route to the next signal. b) No points in the overlap beyond the next signal (Exceptionally, simple facing points may be allowed). c) No directly opposing routes within the route or the overlap. d) No ground frames, controlled level crossings or other equipment with which the signal must be interlocked. The normal aspect of an automatic signal (i.e. the aspect shown when there are no trains present) will generally be a proceed aspect. As the signalman is not directly concerned with the operation of automatic signals, some other facility may be required to stop trains in an emergency. If it is decided that this facility is required, options available are:- a) Individual replacement switches or buttons on the signalman's panel (for all automatic signals or selected signals only) b) Grouped replacement switches or buttons. Each one can replace a group of consecutive signals on the same line to danger) c) A replacement switch mounted on or near the signal, usually requiring a special key to operate. Earlier British practice was to provide replacement facilities on automatic signals in any of the following circumstances:- a) Controlling the entrance to a section in which a level crossing equipped for automatic operation is situated. b) Controlling the entrance to a section in which a tunnel or viaduct is situated. c) Controlling the entrance to a section in which an electrical traction break is situated. d) If not otherwise required, on at least every fifth signal in any section of automatic signals and at any other signal where required for operating purposes. Since the accident at Clapham Junction in 1988, this policy has now changed to one of providing some form of emergency replacement for all automatic signals. On all new installations, this is by means of an individual button on the panel. Automatic signals must be recognisable as such to the driver. In the event of a signal failure and loss of communication with the signal box at the same time, the driver can then pass the signal at danger and proceed at extreme caution to the next signal (prepared to stop short of any obstruction). Some railways identify their automatic signals by a different style or colour of identification plate. SRA practice is to offset the lower signal lights (or marker light) 204mm to the right of the upper signal lights. If clearance constraints make this impossible, and the lights must be in line, a separate plate with a white letter A on a black background is provided. 4.2 Semi-Automatic Signals In many areas signal boxes or local ground frames are provided which provide access to sidings or are not continuously manned. Signals must be provided which permit through traffic to operate when the frame or signal box is unmanned. These signals have to operate automatically for much of the time but must also be capable of control by the local operator. Semi-automatic signals are provided in this case. When the signal box is switched out or the local frame is locked normal, the signal functions as an automatic signal. Once again, the driver must be aware of the correct action to take in the event of failure. A distinct identification plate is used on some railways. The driver must then confirm that the ground frame or local signal box is not in use before treating it as an automatic signal. SRA practice is to provide an internally illuminated "A" indication below the semi—automatic signal (which otherwise has the same appearance as a controlled signal). The "A" is illuminated only when the signal is working automatically. It may also be necessary to divert any signal post telephone circuit to another supervising signal box when the local control is not in use. 4.3 Controlled Signals Any signal other than those described above will be a controlled signal. It must be controlled from a signal-box (other than by Emergency Replacement). It will usually require a lever, switch, button, key or plunger to be operated for each movement. Controls may be provided to allow a controlled signal to operate automatically (i.e. without re-setting the route for each movement). It must be decided whether this should be apparent to the driver. BR practice is not to provide any distinct identification on the signal. The driver will always treat it as a controlled signal, even when working automatically. This is because it is unsafe to adopt any form of "stop and proceed" working where there are points ahead of the signal which may be moved. A signal will normally be "Controlled" if there are points and/or conflicting routes in the route or overlap. 4.4 Signal Identification Plates All stop signals and/or all signals provided with a signal post telephone should be provided with an identification plate. As stated earlier this may be of a different style according to the type of signal (controlled, automatic or semi-automatic). When talking to the signalman, the driver should be able to identify where he is, even if this information is also shown on the signalman's telephone equipment. Other signals (e.g. distants and repeaters) may also be identified for maintenance requirements. Some controlled signals in particular positions may need to be specially identified. For example, the "accepting" signal on approaching an interlocking from a section of automatic signalling is specially plated to remind the driver that he is no longer under the control of automatic signals. 4.5. Signal Aspects Having decided on the necessary position of each running signal on our plan, we must now ensure that we depict each signal to show the correct combination of signal aspects. This will be governed by two main factors:- a) the type and distance of the next signals ahead; whether it is necessary to give warning to the driver to stop. b) whether the signal or the next signal ahead is a junction signal. Any signal that has more than one running route is a junction signal and, as such, must provide the driver with the appropriate turnout aspect and/or route indication when required. For all junction signals on the signalling plan, an adjacent box should be included showing the signal number, description of each mute, its exit signal, and the route indication (if any) displayed. 4.5.1 Aspects for Through Running (Single light) In single light signalling areas a stop signal must at minimum have a head with a red and a green light, with a marker light below. If the next signal ahead is a stop signal at greater than braking distance, a caution aspect must be provided. If the signal ahead is at less than braking distance and the following signal section is also shorter than braking distance, the signal must display also a medium (pulsating yellow) aspect. The signalling plan will show a letter "P" against the signal to indicate that it displays a pulsating yellow aspect. If there is braking distance between the next two signals, the medium aspect is not required. Single light signal heads capable of showing only two lights should be shown on the signalling plan with the actual colours required (R for red, Y for yellow and G for green). Distant signals will usually display caution and clear only. If there is inadequate braking between the next two adjacent stop signals ahead, a medium aspect (pulsating yellow) will also be required. 4.5.2 Aspects for Through Running (Double light) All double light signals have two signal heads, one below the other. The top head is the stop signal and the lower head is the distant for the signal(s) ahead. If the signal is not a junction signal, the upper head will be red & green only. The form of the lower head will depend on whether three or four aspects are required. The, rules are exactly the same as for double light but the aspects are different. The lower (distant) signal head will display red and green only for 3 aspect signalling, red,yellow and green for 4 aspect. The provision of low speed aspects is dealt with elsewhere in these notes. 4.53 Junction Signalling (Single Light) The indication of a route diverging from the main line will be by either a turnout signal (maximum of one route to left and/or right) or a multi—lamp route indicator in conjunction with the main aspect (more than one route to left or right). When the turnout signal is used, the main signal remains at red. All turnout signals must be capable of displaying a caution (three steady yellow lights) and may display a medium turnout aspect (three pulsating yellow lights) when the signal ahead is showing a proceed aspect. When a route indicator is used, the main signal will display a steady or pulsating yellow (according to the next aspect ahead) and the appropriate route indication will be displayed. Clear signals are not given through turnouts. 4.5.4 Junction Signalling (Double Light) The indication of a turnout on a double light signal is by a yellow in the upper signal head. All junction signals therefore require an upper signal head with red, yellow and green lights. The lower signal will still indicate the state of the signal ahead, red if the first signal ahead past the junction is at stop, yellow if the next signal displays a proceed aspect. If a junction signal is set for the turnout (either caution or medium) the previous signal will display a medium aspect, never a clear. Route indicators may be used where two or more turnout routes exist. 4.5.5 Low Speed Signals Section 2.5 has dealt with the main situations in which low-speed signals are used. Firstly, check whether a conditionally cleared caution could adequately fulfil the operating requirements (the overlap should be at least 100m for a conditional caution). Low speed signals are not necessary for normal through running. They should, however be considered in cases where reduced overlaps can aid the regulation of traffic and/or headway for stopping trains and a conditional caution is not appropriate. It is also recommended to provide low speed signals through any area where a track circuit would otherwise control three or more running signals. This is to localise the effect of failures by restricting the number of handsignalmen required in the event of track circuit failures. 4.6 Shunting and Subsidiary Signals Having catered for all running moves, we must now provide for shunting and other non-running movements. Before starting to place signals on the plan, make sure you know exactly what movements are required. Any movements which are not signalled will have to be authorised by handsignals. Handsignalled movements on lines where the majority of trains are properly signalled are disruptive to normal traffic and allow the possibility of human error. Conversely, signals provided for movements which are never used are an additional and unnecessary initial cost to the project. They also represent a continuing maintenance cost and a potential source of additional failures. Although the terms are often used interchangeably, there is a distinction between shunting and subsidiary signals. Subsidiary signals are part of a main running signal. Shunting signals are independent. Subsidiary signals therefore only need a proceed aspect - the main signal provides the stop aspect. Shunting signals must display both stop and proceed aspects. Subsidiary signals can broadly be divided into the following functions:- To shunt from a running line (in the normal direction of traffic) into a siding. To move forward from a running signal into an occupied section. A main route to the same destination may already exist. The provision of both main and shunt routes could assist operations in critical areas during track circuit failures. On a multiple track line, to shunt on to another line in the opposite direction to normal traffic. To move forward to a shunt signal facing the normal direction of traffic. On absolute block lines, to permit a train to pass the starting signal at danger for shunting purposes only. The shunting movement must return behind the starting signal Unless the movement is on to a section of line which is not fully signalled there will need to be an exit signal to limit the extent of the movement. Independent shunting signals can broadly be divided into the following functions:- To shunt between running lines from a position where no main signal is provided. To enter, leave or shunt between sidings. To shunt in the opposite direction to normal traffic. To limit the extent of any shunting movement (including "shunting limit" boards). The diagram below shows examples of some common applications of shunting and subsidiary signals. 4.6.1 Calling-on and Subsidiary Shunting Signals This will be provided where a movement must be authorised to pass a main signal at danger to enter a section which is or may be occupied. An example would be the coupling of two portions of a train in a station platform. Signal 4 on the diagram has a subsidiary provided for this purpose. The usual aspect displayed is a miniature yellow. Some older double light signals display an internally illuminated "CO". 4.6.2 Dead End Signal This will be provided where a movement must be authorised to pass a main signal at danger to enter a dead end siding via a facing turnout from the main line. It displays a miniature yellow light and is offset to one side of the signal post (according to the direction of the movement). Signal 5 is an example of this. 4.6.3 Shunt Ahead Signal Used to shunt ahead of the starting signal and mounted below the main signal. Used on single light signalling only and displays a pulsating miniature yellow light. Signal 7 is provided with a shunt ahead signal to enable long trains to draw forward past the signal before shunting back over the crossover. This type of signal will normally be found in single light signalling areas only. 4.6.4 Dwarf and Position Light Shunt Signals Shunting signals normally have two aspects - stop and proceed. The stop aspect is two red lights and the proceed aspect is a single yellow light. This instructs the driver to proceed at caution. It does not guarantee that the line ahead is clear. SRA uses both dwarf and position light shunting signals. The difference between the two types of signal is the orientation of the lights. The choice of signal type will depend mainly on lineside clearances. On a position light signal, the two red lights are side by side, the yellow light is above. A dwarf signal has the three lights vertically arranged; the red lights are at the top and bottom with the yellow light between. Route indications are provided where required, particularly where wrong line movements are signalled. 4.6.5 Shunting Limit Boards Effectively a shunting signal fixed at danger, a shunting limit signal faces in the opposite direction to normal traffic and is used to limit the extent of a wrong line shunting movement. An example is shown on the down line. This would enable trains to shunt out of No. 1 siding on to the down line before proceeding forward. Without the board there would be no signal to prevent the wrong line movement continuing indefinitely on the down line. 4.6.6 Facing (or Preset) Shunt Signals Occasionally, shunting movements in the normal direction may be required to start from a position where a running signal is not provided. Such a shunt signal must therefore be passed by normal running movements. To avoid the driver seeing a yellow light after he has just passed a main signal showing clear (and possibly braking unnecessarily) "facing" shunt signals are provided with an additional green light to show clear when the previous main route is set past the shunt signal and the signal is showing clear. Signal 55 is a facing shunt signal. 4.6.7 Point Indicators Point indicators should be provided on any points (whether facing, trailing or catch points) where the driver is responsible for observing the position of the points before proceeding over them. The points will usually be hand worked, as shown in siding 1 on the example. Where regular shunting takes place without the need for the signalman to set the route for every move, point indicators will be displayed. These will be selected by a separate button on the signalman's panel. This is preferred to providing two shunt signals with opposing locking removed as it avoids the possibility of two trains approaching each other both under proceed aspects. 4.7 Trainstops SRA provides trainstops on most of the Sydney metropolitan area. Double light signalling is normally provided. All electric multiple unit trains are provided with tripcock equipment which will apply the brakes if a train passes a raised trainstop. The trainstops are provided at each main stop signal and in certain other locations (e.g. exits from depots and sidings) to prevent a rear end collision with another train. If the signal is at danger, the trainstop will be raised. A train irregularly passing a signal at danger will be tripped and brought to a stand within the length of the overlap. Where trainstops are to be used, the engineer must ensure that the length of each overlap is adequate for emergency braking at the highest speed at which a train is likely to pass the signal. Obviously the trainstop cannot ensure total safety if all trains are not fitted but it can make a major contribution to safety in areas where trains regularly run at close headways. An important part of the preparation of the signalling plan is therefore to decide where trainstops are to be positioned. This is closely associated with the calculation of overlaps. It may often be more important to accurately position the overlap for track circuit clearance purposes, then work back to the position of the signal and the trainstop. A low speed signal tells the driver that there is little or no overlap beyond the exit signal. Running speeds will be low (normally less than 35km/h). The low speed overlap will be based on the passing speed of the low speed signal. However, the driver could fail to brake, or even accelerate after he has passed the low speed signal. This would leave an inadequate low speed overlap. Intermediate trainstops are therefore often provided between a low speed signal and the next signal, to be lowered only after sufficient time has elapsed for the train to have reached the trainstop at or below the correct speed. The following general rules therefore apply to the positioning of trainstops:- A trainstop is required at all stop signals. It must always be on the same side of the line. SRA provides trainstops on the left hand side, London Underground and British Rail use the right hand side of the track. Additional trainstops may be required on the approach to stop signals with a reduced overlap where a speed reduction has already been enforced at a previous signal. As an example, a low speed signal reading into a station platform could have a low speed aspect to allow early entry of following trains. The overlap associated with this may be 100 metres or less, even reducing almost to zero. To ensure that a train does not accelerate to a speed which would render the overlap inadequate, an additional trainstop is provided on the approach to the exit signal after the low speed signal. The lowering of this trainstop is timed to trip a train which is running above the permitted speed.  The positioning of trainstops therefore has to take account of the braking and acceleration characteristics of the train and the length of the overlap. The calculation can become very complex so a simple example is used here to illustrate the possibilities. In the following diagram, the two stop signals are 200 metres apart. For headway and/or junction clearance purposes, it has been decided that only a 50 metre overlap is available beyond the second signal. We will assume that the train passes the first signal, displaying a low speed aspect at 27 km/h or less (otherwise it would have been tripped). This example will assume a typical service braking rate of 0.9 m/s 2 , an emergency braking rate of 1.4 m/s 2 and an acceleration rate of 0.55m/s 2 . These are typical of those which have been used for SRA signalling for electric multiple units although the actual performance of the trains which will use a line must always be confirmed, and gradients taken into consideration. The train should under normal circumstances brake to a stand at signal 2 along or below curve A. The trainstops should ensure that the train will come to a stand within the overlap, should the driver fail to take the correct action to control his train. There are various possibilities which may arise. The signal engineer must decide whether to allow fully for all of these or whether circumstances will permit some relaxation. After passing signal 1 at the permitted speed (27 km/h in this example) the driver could totally fail to brake. Even worse, he could accelerate after passing signal 1. We could assume either no acceleration, acceleration due to gradient only or acceleration under full power. Whichever is chosen, the overlap should be greater than the emergency braking distance from signal 2. If this is not the case, an intermediate trainstop (labelled ITS) must be provided which should be timed to lower just before the train reaches it on a normal service braking curve (point X on the diagram). Curve D shows the effect of this trainstop on a train accelerating under full power. With the intermediate trainstop having been passed at the correct speed (lowered before the arrival of the train), the train could then accelerate at full power towards signal 2. In this case the trainstop at the signal will ensure the train stops within the overlap. Curve B shows the likely speed profile of the train in this situation. Even with these safeguards it is possible that a train could stand just past signal 1 on the timing track circuit for the intermediate trainstop. The trainstop would lower after the prescribed time interval and the train could then accelerate under full power towards signal 2 without the protection of the intermediate trainstop. It will be seen from curve C that the train will overshoot the overlap, passing the overlap joint at up to 9 m/s. To overcome this, the length of the timing track circuit for the intermediate trainstop must be limited such that an accelerating train could pass signal 2 at a speed no higher than that possible on curve B. Alternatively, an additional intermediate trainstop could be provided to check the train speed at an earlier point. SRA practice in open (i.e. above ground) areas is to allow some margin for possible acceleration but not the deliberate full acceleration of curve C. In tunnel sections where the driver's perception of speed and distance may be affected, the positioning of trainstops and their associated timing track circuits should cater for all possibilities. It should be noted that due to the introduction of newer trains with better acceleration characteristics, the protection provided by certain older sections of signalling is now reduced. It will still protect against most normal occurences other than the deliberately malicious driver intent on overriding the protection of the signalling equipment. Typical distances for open areas are as follows:- For following trains and overlaps less than 50 metres, the intermediate trainstop is positioned 100 metres from the end of the overlap with a timing track circuit between 80 and 220 metres in length. For overlaps clear of fouling movements, the overlap should be at least 100 metres and the intermediate trainstop 200 metres from the fouling point. In addition, any previous signal whose full overlap extends beyond the fouling point should be conditionally cleared to caution. This arrangement is not recommended where signal spacing exceeds 500 metres. 5. POINTS AND CROSSINGS Although, in general, the siting of points and crossings on an existing railway will be dictated by permanent way design considerations, it is left to the Signal Engineer to determine the operation of the points. Furthermore, the Signal Engineer may require additional trapping protection to be provided on occasions and such cases must be referred back to the Permanent Way Engineer (Way & Works Engineer on SRA). In the case of combined track remodelling and resignalling projects, it is sometimes possible to provide simpler or improved signalling controls by minor alterations to the track layout. Close co-operation between the Signal Engineer and the Permanent Way Engineer is essential if the optimum results are to be achieved. 5.1 Position and Numbering of Points Any set of points will be defined as lying in its Normal position for one route and its Reverse position for the other route. The Normal position of the points will be shown on the signalling plan as follows:- A similar convention applies to switched diamond crossings, if used. Points should be numbered in such a way that any point ends required to work simultaneously carry the same number. To localise failures, it is not advisable to number more than two ends to work together. In addition, Solid State Interlocking (SSI) equipment is normally only configured to operate single and double ended points, although in certain circumstances three ends can be accommodated. For control purposes, each end has to be identified separately (A or B) but this may not need to be shown on the signalling plan. A convention must be determined for identifying A and B ends (e.g. A end nearest control centre or A end at lowest reference distance etc.) and strictly observed. On SRA a down train will meet the A end first. 5.2 Ground Frames Ground frames control infrequently used points, usually outside interlocking areas. Although referred to as ground frames, they may equally well be locally operated control panels. In its most common form the ground frame consists of just 2 levers, the point lever and a release lever (which will also work the F.P.L. if the points are normally facing). Movements over the points during shunting are usually controlled by handsignal, although extra levers may be provided to control or slot signals which the train must pass during shunting. Note the use of separate releases where the ground frame controls more than one function. Instead of providing an electric lock on the release lever, a separate key is electrically released when the signalman operates the release button. This key is then used to release the ground frame release lever. It remains captive until the ground frame is normalised and can then be returned to the instrument to give back the release. 5.3 Trapping Protection It may be necessary to request trap points (normally known as catch points on SRA) to be provided at certain locations: At the exit from sidings, where they lead on to running lines, catch or trap points must be provided to prevent an unattended vehicle running away or a shunting movement overrunning and fouling the running line. Where a full overlap cannot be obtained and movements are required to closely approach a converging junction, catch or trap points leading away from the running line can be used as an overrun in place of the normal overlap. On railways where a distinction is made between passenger and non-passenger lines, trap points may be used where the non-passenger line joins the passenger line. Where trap/catch points occur in track circuited lines many railways employ a track circuit interrupter to ensure a derailed vehicle which is still fouling the track, although not standing on the rails, remains detected. The track circuit interrupter is normally insulated from the rail on which it is mounted and bonded in series with the opposite rail. 6. TRACK CIRCUITS Track circuits shall be provided in a manner which permits maximum flexibility with minimum expense and complexity. 6.1 Overlaps Running signals should, in general, be provided with separate berth and overlap track circuits, the berth track circuit terminating immediately beyond each signal. This will ensure the signal is replaced to danger at the earliest opportunity after the train passes. Where more than one overlap is required, a joint must be provided at the end of each overlap. Calculation of overlaps has already been covered in the earlier sections dealing with the positioning of signals and trainstops. However, SRA practice on overlaps is summarised below. 6.1.1. Where Trainstops Are Not Fitted The overlap is a margin to allow for braking errors. There is no positive means of stopping the train if a driver completely misses or misreads a signal. As such it is an approximate distance based on experience, rather than one which has been calculated on any scientific principle. The standard SRA overlap is 500 metres. This may be smaller or greater than the actual braking distance. Where speeds are low, this is sometimes reduced. Recommended overlap lengths are:- Speed above 80 km/h 500 mtrs 60 - 80 km/h 400 mtrs below 60 km/h 300 mtrs For low speed movements entering passing loops on single lines 200 mtrs 6.1.2 Sydney Metropolitan Area (Open Sections) As trainstops are fitted, the overlap must be based on emergency braking distance for the prevailing speeds and gradients. The following example shows how this may be calculated. To simplify the calculation when dealing with gradients it is often easier to express the braking rate as a percentage of the acceleration due to gravity (g = 9.8m/s 2 ). Braking distance = v 2 /2a (where a = braking rate, v = train speed) = 100v 2 /2g(%B + %G) %B = Braking rate as a percentage of g %G = Gradient as a percentage (down gradients negative) If the line speed is 25 m/s, the braking rate is 10% and the gradient is 1% down, the emergency braking distance and hence the overlap will be:- 100 x 25 2 / 2 x 9.8 x (10 - 1) = 62500 / 176.4 This would give a minimum overlap of 354 metres. This would probably have to be increased by a suitable margin to allow for less than 100% braking performance (e.g. some brakes isolated, wet or greasy rails, delay time for brake application). Where available, braking tables or curves should be used. If the full overlap is foul of junctions or station platforms, a reduced overlap should be considered. The train speed would have to be suitably reduced by a low speed or conditional caution aspect at the previous signal. 6.2 For Points and Crossings The positioning of track circuit joints to prove clearance will depend on the dimensions of the rolling stock in use. One must first determine the difference between the maximum vehicle width and the width of the widest vehicle in service. The position must then be found where the rails leading away from the crossing are at least this distance apart (normally adding a small safety margin). At this point, the extreme ends of vehicles on the adjacent tracks will not be foul of each other. Measuring away from this position, the joint must be located at a distance greater than the maximum end overhang of any vehicle. This is obtained by measuring from the centre of the outer axle to the extreme end of the vehicle. Track circuits should allow maximum flexibility of use of the layout. In particular, where the track layout permits parallel moves, the signalling must not prevent them. Joints should be positioned to achieve the earliest release of points after the passage of a train consistent with safety, economy and practicality of installation. Example 1 Joint A allows simultaneous moves over both ends of crossover normal. Joint B allows points to be moved as soon as train leaves points. Joints C, at clearance point, allow movements across crossover with Tracks X and Y occupied. It is common on plans to place joints C opposite the tips of the points. Example 2 Joints A allows parallel moves. Joints B allow points to be freed as soon as junction cleared. Joints C are set back at clearance point. These may also be the overlap joints for signals approaching the junction. Joint D will be dependent on factors other than the requirements for operation of the junction, eg. the position of the protecting signal. 7. NUMBERING OF SIGNALS, POINTS AND TRACK CIRCUITS To enable all signalling controls to be specified, each signalling function must be uniquely identified. It aids design, testing and fault location if this is done in a logical and orderly manner. In particular, confusion is avoided if different types of functions are numbered in different number or letter series. The main functions which need to be numbered are:- Main Signals Shunt Signals Points Track Circuits Ground Frame & other releases Separate number series should be provided for each type of function (points, signals etc.). Main and shunt signals may be numbered in the same or separate series. Lines for each direction of traffic are normally designated UP and DOWN. Signals reading in the Down direction normally carry odd numbers with the lowest number at the Up end of the control area. Signals reading in the Up direction normally carry even numbers, again with the lowest number at the Up end. Points will be numbered with the lowest number at the Up end of the control area. Where possible suitable gaps should be left in the numbering sequences in anticipation of future alteration. Distinct branches should be numbered in separate series. Historically, several different conventions have been used for identification of track circuits. Each has advantages and disadvantages. One common method is to use a simple numbering sequence. The disadvantage of numbers is that, on a large installation, very large numbers or duplicate number sequences need to be used (with greater risk of errors in design and testing). Another alternative which has been used is to number track circuits based on the distance along the line. This results in track circuits in one locality having long and very similar numbers. Again confusion and errors may result. SRA uses a system based on the signal numbers. The first track past signal 5 would be 5A, the next 5B and so on. The suffix E is not normally used. The BR standard is now to use letters. Each track circuit indicated to the signalman should be identified using two capital letters, arranged alphabetically in a logical sequence. Letters I and O are not used. Where a number of track circuit sections have a common indication they should have the same identity plus an individual suffix number, eg. AA1, AA2 etc. This arrangement is simple but does not give any indication of the relative locations of tracks and signals. 8. EXAMPLES A few examples are now given of some of the more commonly found track layouts and suggested arrangement of signals. They do not cover all situations. In practice, different requirements will conflict. The signal engineer must resolve these conflicts in the most effective and economic manner 8.1 Junctions The logical arrangement at a junction is for the protecting signal to be as close to the junction as possible. For diverging movements this ensures that trains are not checked too far from the junction, while for converging movements it reduces the chances of trains being checked due to conflicting moves on the junction. The signal next in rear of the junction cannot be cleared unless the section is clear up to the junction signal, and the overlap beyond. It is preferable that the overlap is not fouled by conflicting moves, so ideally the signals protecting the junction should be placed overlap distance in rear of the junction. If large overlap distances make this impractical, a reduced overlap clear of the junction should be considered. In this case the signal in rear should have its full overlap clear of the junction. 8.2 Station Platform with Loop It is usually desirable for headway reasons to site a signal at the end of the platform. This provides a platform starting signal and also protection for any level crossing at this point. In situations where there is no platform starting signal, there is a risk that a station stop will divert the driver's attention sufficiently for him to forget the aspect displayed by the previous signal. After restarting his train, he could approach the next signal at an unsafe speed. In the above example, signal A is located at full S.B.D. from signals B & C. For main line running this is satisfactory, but for a move into the loop the time taken for the train to slow to 25 km/h over the crossover may adversely affect the headways. A better arrangement is shown below : Signal A is moved closer to the turnout. If this results in inadequate braking distance from signal A to signals B & C, signal D must be a 4 Aspect signal. Signal A would only need to be 4 aspect if the signal ahead of C was less than braking distance. 8.3 Terminal Stations The station throat track capacity must be at least double that of the approaching line. This is because some arriving and departing movements will completely block all other routes. The signal reading into the platforms should be as close as possible to the plaforms. If this signal does not have an overlap clear of points, the signal in rear should do so to allow trains to approach unrestricted. Signal spacing on the approach to the station should be as close as possible consistent with standing room and headway requirements. The first signal leaving the station should be as close as possible (whilst retaining necessary train standage clear of the pointwork), to allow the best possible aspect on the platform starting signals. The standage requirement may have to be reduced to maintain adequate line capacity on the departing line. The platform entry signal 3 exhibits stop and caution aspects only for running moves. Buffer stops are equivalent to a signal permanently at stop. Subsidiary shunt signals are provided to enable trains or locomotives to enter occupied platforms. If wrong line shunting is required, a shunt limit board must be provided at a position which permits adequate standing.

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