CONTENTS 

1. Introduction

2. Push Button Interlocking

3. Point Circuits

4. Route Locking

5. Signal Aspect Controls

6. Route Releasing

7. Overlaps
8. Preset Shunt Signals

DISCLAIMER :

THIS ARTICLE DOSEN'T REFLECT THE CURRENT NSW PRACTICE AND MORE OVER ROUTE RELAY INTERLOCKING IS GETTING REPLACED WITH COMPUTER BASED INTERLOCKING.THIS ARTICLE IS PURELY FOR INTERLOCKING STUDY PURPOSE  AND HELP TO GAIN KNOWLDEGE ON INTERLOCKING .CIRCUITS USED HERE SHALL NOT BE  USED FOR OPERATING LINES WITHOUT CHECKING   LATEST PRACTICE .THIS IS BASED ON AUTHORS UNDERSTANDING OF THE NSW INTERLOCKING 

1. INTRODUCTION

1.1 Development of Relay Interlocking Systems

The earliest interlockings were mechanical, between the levers of a frame but, with the development of block systems and track circuits, electrical controls were added. Staffing levels could be reduced by displacing a number of small signal boxes by fewer, larger installations. Fully electrical interlocking systems became necessary to control power operated signalling equipment. Initially, miniature lever frames were used. There are however many advantages in using a control panel incorporating buttons and switches into a diagrammatic representation of the track layout.

With panel operation, a totally relay based system was required. Many suppliers produced their own different systems which, although different in detail, followed similar principles of design and operation.

The signalman's control device was usually a button or a switch. Unlike the lever, it was free to move at any time. The relays now provided the security of the interlocking system. Additional indications had to be provided to show the signalman the state of the interlocking.

During a period of substantial investment in signalling modernization in the 1970's, design, installation and testing resources were in short supply. British Railways  invited major suppliers to offer standardized interlocking systems which could as far as possible be factory wired and automatically tested. The contractors met the need with modular or "geographical" systems. The two main systems adopted, the Westinghouse "Westpac" and the AEI (GEC later acquired by Alstom) systems underwent several stages of development and were widely used.

These systems certainly provided for very quick installation but there were disadvantages. The most important was that each development of a system was incompatible with its predecessors.

The two systems were very different in their design philosophy. Westinghouse adopted a "one set per function" approach which led to high levels of redundancy, whilst GEC provided several sets combined as necessary for the function present. This approach increased substantially the quantity of inter-set wiring.

Due to rising costs and problems of spares and modifications, geographical systems have fallen in popularity.

State Railway Authority of NSW (later RailCorp, now TfNSW ) never adopted any form of geographical interlocking but has for some years purchased interlockings built to its own set of standard circuits. BR(British) also developed a set  of typical circuits for free-wired interlockings which  in operation  would appear similar to the geographical systems and adopt a high level of standardization of circuit design.

These were incorporated into a BR Specification (BR850) which is probably one of the most standardized, and probably the last, relay interlocking specification in widespread use.

On BR the use of route relay interlocking systems has now been superseded by solid state interlocking for all new work.

As most route relay interlocking systems follow similar design principles and methods of operation, the State Railway Authority of NSW(Railcorp/Currently TfNSW) circuits will be used throughout for reference.

NOTE:- This article is purely written for study purpose and shall not be applied without checking current practice 

1.2 Relay Types 

Various types of relay are used in the interlocking. As well as neutral relays, slow to operate, slow release and magnetically latched relays are used. It is important that the operation of each type is understood, and circuit symbols recognized, in order to follow the operation of the equipment. In particular, latched relays will remain in the last position they were moved to.

1.3 Diagrams 

All the circuit diagrams used for reference are based on the NSW (New South Wales. Australia) standard circuits.

1.4 Variations 

In practice, different installations may vary slightly in detail of circuit design. For example, full overlap swinging facilities may not always be provided.

It is important to understand the relationship between the control tables and the interlocking circuits. The circuits must always be designed to function in the manner described by the control tables.

1.5 Relay Names and Functions

1.6 Control Panel Symbols

Figure 1 to 4 below shows the detail of the panel push buttons. Of particular note to this course is the directional arrowhead on push buttons giving information about their use as the start (entrance) or finish (exit) of a route. The button surround may be of a different color according to the button function (main or shunt) and direction of traffic (up or down).

 

2. PUSH BUITON INTERLOCKING 

2.1      Operation of Entrance-Exit (N-X) Panel 

In route control system of signaling, a route is set and the signal leading over it cleared by the signal man operating two pushbuttons which are located on a control panel .The first button operated is at the commencement(Figure 1)  of the route and the second button at the finish(Figure 2) of the route .The finish button is generally at the next signal applying to the direction of traffic being dealt with, but in the case of a route which leads into a section, siding or terminal road the finish button is located in the section, siding or terminal road.

Providing all conflicting routes are normal the push button operations are registered and any points in the required route which are not in the correct position will operate, then providing the line is clear to the clearing point, the signal will exhibit a proceed indication.

If a conflicting route is set or a previous train is passing over points within the route and the points are out of position for the next movement, the button operation is not registered, and it will be necessary for the signalman to again operate the buttons when the route is free.

To clear the next signal the last button operated which represented the finish of the previous route is again operated and acts as a commence button for the next route. A second button is then operated to locate the finish point for this route.

After the passage of a train, or if it is required to cancel the route, without the passage of a train, the commence button for the route must be pulled to restore the route to normal. A signal cannot clear for a second train unless the route is normalized and then set again.

If a button has been pushed as a commence button and for any reason the route which was to have been set is not required, the commence action may be cancelled by pulling that button.

Only one button may be effectively pushed at a time and when operating a button as a finish button it should be depressed for approximately one second.

The entrance-exit (N-X) push button panel is the standard type used by British and NSW Australia, and widely used elsewhere. The method of operation to set a route is: -

Press the entrance (start) button and release it. The button will flash to indicate it is the selected entrance.

The next button pressed must be the exit or finish of the route, and provided the route selected is both valid and available, the route will be set and locked, and white route lights will indicate the extent of the route set. The entrance button will display a steady white light.

If an invalid exit or an unavailable route is selected, the route setting will be aborted.

Because some buttons may function as both start and finish buttons (Refer Figure 5), and since the start signal may have several valid routes, each with a different finish, a constraint is imposed that only one route may be selected at a time. Where this would be over restrictive, several independent push button interlockings will be provided, one (or more) for each signalman's control area. In below figure 5, button for Signal 2 is a finish button for route from Signal 1 and is a commence button for route leading to Signal 4 and 6

 

LEGEND 

Note: When relay is energized (Up), front contact will be made(close) and back contact will be open, similarly when relay is de-energized (Down) front contact will be open and back contact will be made (Closed)

Some relays will be Normally Energized (EG: USR, ALSR, Track Relay), whereas some relays will be Normally De Energized (EG: DR Relay for Control Signal to show green aspect)

Non Vital Relay contacts are same ,but dot  on each end of  armature ,thats how we identify 

2.2            Push Button Relays 

All push buttons on a panel have three positions, middle (denoted 'M'), pushed (denoted 'F' - meaning "from" the operator) and pulled (denoted 'T' - "towards" the operator). The button is sprung to return to the middle position after it is either pushed or pulled. Depending on the exact function of the button, relays will be provided to repeat the relevant positions or combination of positions of the button. Refer figure 5 for the most common circuit. To set a route from No .1 Signal to No.3 (M) Signal, button No 1 (for Commence) is pressed, energizing 1(FR) relay

 

The diagrams show these relays wired direct to the panel button. This is the normal arrangement for the local interlocking at the control centre. For other interlockings, a remote-control link, normally TDM, is employed. Refer Figure 7. Any Telemetry such as Kingfisher also could be used. Traditionally control system is non vital system used for route request and indication of equipment status back from the track.

 

2.3 Push Button Checking Circuit 

At various stages in the route selection process, it is necessary to prove that only one button is being pressed. This is accomplished using the (R)PR circuit sample shown in Figure 8

The (R) PR provides the interlocking between buttons to ensure only one button operation is registered at a time.

1(F) R contacts make, energizing 1(R)PR relay. This lifts 1(R)PR contacts cutting out all other (R)PR Relays

Back contacts of all following (R) PR relays are included in the negative side of each (R)PR relay to prevent it picking up if its button is operated (F) R energized whilst any following (R)PR is already energized. Thus, ensuring only one button operation will be registered at a time.

2.4 Pressing the Commence Button 

The interlocking will store the signal selected as the start of the route by holding the appropriate CeR up. Refer Figure 9

The CeR initiates the commence operation to set a route.

1(R)PR contact makes, energizing 1CeR relay. 1CeR stick contact is made and holds 1CeR relay energized when 1(R)PR drops out (ie when commence button No. 1 is released) via 1(N)R & FnJP2R contacts.

1(N)R contact is a route cancelling contact which energizes when a button is pulled and the FnJP2R contact (2nd repeat finish normally closed relay) forms part of the timing cycle circuit. These circuits will be covered later.

As 3 buttons can be either a commence or finish button, 3FnR (Finish Relay) is proved de­ energized in its commence relay circuit.

2.4.1       Machine in Use

The MUR sets circuit operation to ensure the next button operated initiates the finish operation for the route.

1CeR contact picks up and energizes the MUR relay whose contact turns MUR indication light on (flashing red). The FnPR contact ensures that the MUR is held energized to prevent the MUR from dropping out if a finish button is held longer than one second this would change the button from finish to a commence function (Refer Figure 13).

In Figure 11 MUKR is a repeat of the MUR and the FnPKR is a repeat of FnR, Refer  Figure 12 This gives a flashing red or green indication on the console which indicates to the signalman whether the machine is in use or finish.

When FnJP2R relay drops out 1CeR relay de-energizes, thus extinguishing the MUR light. Refer Figure 14.

2.5  Pressing the Finish Button

The finish of the route will be identified by one FnR energized. The combination of CeR and FnR uniquely identify the route. Refer Figure 7 above. To select destination (finish) in this case there is only one possible destination from Signal 1 which is No. 3 signal. Pressing button 3(M) for finish, energizes 3(M) (F)R.

The (FM)R relay provides for emergency replacement of a signal after the route has been set.

Refer Figure 12 for Finish Relay (FnR) Circuit

 

The FnR initiates the finish operation to complete the route being set. When the 3(M) (R)PR energizes, and because the MUR relay is already held up via No. 1 CeR as shown Figure 10,

3FnR relay energizes for the period of time that the button is held in. As No. 3 button can be either a commence or finish button, both commence relay functions, 3 & 3 (S), are proved down in its finish relay circuit. This holds the FnR de-energized during the commence function when the MUR has picked up and while the (R) PR is still energized.

2.6 Push Button Circuit Normalization

Approximately 1 second is allowed before the push button circuits are normalized, to permit setting of another route. This prevents preselection by the selection of the commence and finish signals before the route is available. Figures 13 & 14 show the normalizing circuits.

The FnPR (finish repeat relay) initiates the timing sequence for a route to set. Refer Figure 13 for Finish Repeat Relay Circuit.

When 3FnR contact makes, and energizes the FnPR relay the timing cycle, of approximately one second commences and, in this period the route must be capable of setting. If the route is not capable of being set within one second of the signalman operating the finish button, the action is not registered and the entire operation for setting a route must be commenced again.

The FnJP2R and the stick function of the FnPR maintain the FnPR energized if the finish button is released before the timing cycle is complete to allow the timing cycle, once commenced to once completed.

The FnJR & FnJPR's (finish timing relays) provide the timing sequence initiated by the finish function.

The FnPR energizing starts the timing cycle through the slow to drop relays FnJR, FnJPR & FnJP2R.

When the FnJP2R relay drops out a number of things occur:

1. The CeR relay is dropped out via FnJP2R contact in the stick path circuit as shown on Figure 9.
2. The MUR relay is dropped out via the CeR contact dropping out in circuit as shown on figure 10
3. The FnPR relay drops out if the finish button is released in circuit shown on figure 13, via the FnJP2R de-energized in its stick circuit.

This timing sequence provides the non-storage feature of the system. (i.e routes cannot be pre­ selected until a train has vacated the route).

2.7 Effect of Pressing the Wrong Type of Button 

Pressing a finish only button at the start of route setting will have no effect because the signal does not possess a CeR. The circuits will normalize as soon as the button is released.

Pressing the incorrect finish button will result in no route being set as the FnR and the CeR previously selected will not both appear together in the same route setting circuit anywhere in the interlocking.

2.8 Indications Panel 

To assist the signalman indication lamps are provided to show "machine in use" or "machine finish" to advise him of the current state of route setting.

On many BR interlockings, the technician is provided with the facility to hold the equivalent of the FnJP2R energized to assist fault finding. This enables him to test circuits which otherwise may only be energized very briefly. This facility must be used with great care, and with the agreement of the operator, as it will inhibit the setting of any other route in the interlocking.

2.9 Route Control System-Route Setting

Having now dealt with the operation and circuits associated with the control panel switches, we will now cover the operation and / or function of each relay, for the setting of the points to the correct position, locking them, setting the required route, and clearing the appropriate signal leading over that route which will include route locking and approach locking etc. Refer Figure 18 & 19

2.10 Route Setting

When a button is pushed to select the commencement of a route (R)PR energizes & providing that no other button has been pushed a contact on the (R)PR closes the circuit for the 'commence relay' CeR, (Refer Section 2.4). A front contact of the CeR then energizes the 'machine in use' relay MUR (Refer Section 2.4.1). The MUR energized opens the pickup circuit for all CeR's & this determines that the next button to be pushed will function as a 'finish' button. The CeR for the button operated is held energized by a stick circuit which includes a front contact of the finish timing relay FnJR, back contacts of its own (N)R relay & its own front contact.

When the next button is operated to define the finish point of the route to be cleared, its  (R)PR is energized & because the MUR is up, the finish relay FnR (Refer section 2.5) for that button will energize for the period of time that the button is held in. A circuit is provided for the MUR via a front contact of FnPR, the finish relay's repeat relay, to prevent the MUR from dropping out if a finish button is held for longer than one second. (This would change the button from a finish to a commence action).

Contacts of the CeR & FnR relays in series are utilized in the negative side of the route NLR delatching coil and drive the relay down this in turn closes the circuit for the route RUR and providing all locking and track circuit conditions are satisfactory the route will set and its signal clear. (Refer Section 2.12).

The commence relay CeR and FnR (finish relay) remain energized for approximately one second after the finish relay has operated but long enough to allow the route NLR to delatch and the RUR to energize.  This sequence provides the non-storage feature of the controls.   That is, if the route is not capable of being set within one second of the signalman operating the finish button, his action is not registered & he must operate the buttons again when the route is free.

The method of obtaining the one second timing period is as follows. When a finish relays FnR energizes, its front contact completes the circuit to the finish repeat relay FnPR (Refer Section 2.6) & a back contact of the FnPR opens the circuit to the finish timing relay (Refer Section 2.6). The three finish timing relays are slow-release relays & approximately one second after the FnPR has energized the FnJP2R opens its front contact to break the holding circuit for the commence relay network (CeR). (Refer Section 2.4).

A stick circuit is provided to hold FnPR energized until FnJP2R is de-energized.   This ensures that if the finish button is released before the timing cycle has been completed, the FnJP2R will still release & cancel out the CeR.

When the commence relay releases & the finish button has been released the MUR releases, and with the timing relays energized & all button relays are in their normal position the system is ready for another route to be set or for another attempt to be made to set the same route.

2.12 Route Normalizing

The (N)R Relay controls(Refer Figure Below) the normalizing of the appropriate route NLR, and when energized, drops the route reverse relay (RUR), and latches up the route normal relay (NLR).

Refer below figure 20  for a typical (N)R Circuit 

Figure 20 Normalising Relay 

When No. 1 button is pulled, 1(N)R is energized and is held up by a back contact of the route NLR which is to be normalized, a back contact of 1 CeR and its own contact.  The stick circuit will maintain 1(N)R energized until the route NLR circuit is completed by the signal returning to stop or the approach stick energizing as the case may be. The push button can therefore be released immediately after it has been pulled. Once the route NLR has latched up, its circuit is opened by the (N)R relay dropping & it is held in the up position by its magnetic latch.

The back contact of the CeR in the (N)R stick circuit permits a signal to be recleared, if required, after it has been cancelled but the route has not normalized due to approach locking.

A back contact of the (N)R relay is wired in the stick circuit of the relative CeR relay & this allows a button which has been incorrectly pressed as a commence button to be cancelled by pulling the button.

2.12 Checking Route Availability & Validity 

The Commence and Finish selected by the signalman, stored as CeR and FnR, must now be checked for validity - physical route possible - and availability. Normal lock and reverse route relays are provided for each possible route in the interlocking.

These circuits comprise two parts. To the right of the relays (in the negative feed) are the relay contacts which respond to the push button circuits. The validity of a route is proven by the presence of a circuit with the correct CeR and FnR combination. Refer Figure 15

The availability of the route is checked via the positive feed where all locking is proved. An example of a route with points is shown on Figure 16. The points must be in the required position (NLR or RLR up) or free to move (WZR up).

Providing the route is available the NLR will unlatch and allow the RUR to pick and stick. This operation must complete within the 1 second before the push button circuits normalize.

In the event of the route not being available at the time of selection, or within one second, the push button circuits will normalize. The selection is not stored until the route becomes available but can only be acted on at the time of selection.

2.12.1 Route Lock Relay (NLR/RUR)

Each route in the interlocking from signal to signal or from signal to section, siding or terminal road has a RUR to set points and clear the entering signal and a NLR which proves the route normal and is used in locking conflicting routes.

The route lock relay circuits for No. 1 route are shown on Figure 15. The route RUR and NLR circuits are electrically interlocked with each other. Thus, 1NLR back contact is in series with 1 RUR operating coil and 1RUR back contact is in series with 1 NLR operating coil.

The NLR is a magnetically latched relay and remains latched in its last operated position. It has two coils, one to latch the relay up, and another to latch the relay down. The operation is described on section 2.12.2 

The route NLR when latched up is used to release conflicting routes, and proves that: -

• the signal has returned to stop

• the signal is not approach locked

• the route RUR is de-energized & is therefore not capable of setting points or clearing the controlling

The route RUR when energized proves that the route NLR is latched down thereby checking that all conflicting routes and points are locked prior to the route setting.

The interlocking between routes is carried out in the positive leg of the RUR relay in accordance with the locking table for the interlocking. If a route requires that certain other routes must be proved normal before that route can set, then normal contacts of the conflicting route NLR's are included in the positive side of the RUR for the route concerned.

The interlocking between routes and points is also carried out in the positive leg where a contact of the points NLR or RLR is included and qualified by a contact of the WZR for the points concerned if the points are out of position but are free to move.

Note: Positive leg of the route locking relay circuit included the interlocking function (Safety Function) and negative leg of the relay circuit included the route selection (Non Safety Function)

Refer Figure 17 for sample control table. Routes and point conditions reflected are the logic for the positive limbs of the RUR/NLR circuits referred in Figure 15/16.

Eg: Front contact of Route 9C NLR will be in series with 1NLR and 9BNLR, where 1NLR is qualified(parallel) by point 103NLR and 112 RLR and 9BNLR qualified by Point 112RLR. This will be in series with Points required/free to move Normal or Reverse

2.12.2 Route Lock Relay Circuit Operation (NLR/RUR)

Refer Figure 15, when the commence and finish push buttons have been operated to clear No 1 signal, 1 CeR and 3FnR contacts will be up together as described in Section 2.4 & 2.5. This drives 1 NLR magnetically latched relay down and closes 1NLR contacts.

This action then completes the circuit for 1RUR relay to energize via 1 CeR and 3 FnR contacts.

Front contacts in the negative leg of 1 RUR circuit then close, and hold the relay energized via 1(N)R and 1 (FM) R normalizing contacts after 1 CeR and 3 FnR have dropped out at the completion of the timing cycle.

The route will remain set until the commence button at No 1 signal is pulled, which will pick up 1 (N) R contact and open 1 (FM) R contact.

If the ALSR relay is de-energized, ie, the route is approach locked, the route cannot be normalized to release the interlocking. However, it may be re-cleared for the train to proceed.

3 POINT CIRCUITS 

3.1 Principles of point operation

 Points may be called to operate by one of two methods: -

a) The setting of a route requiring the points to be moved using route setting  buttons, or -

b) The operation of a point key (lever) on the panel.

At the time of calling, the points must be free of locking in their present position. Points may be locked by route locking, track circuit occupation, the point key having been turned, or another route having been set.

The points must be free at the time of selection, and the selection must not be stored until the points become free, (anti-preselection).

In the event of a power failure the last legitimately selected position of the points should be held, and, on restoration of the power, the points should not be called to another position due to the random recovery times of different relays within the interlocking.

 3.2 Calling the Points 

Figure 21 shows a point Lock relay (NLR/RLR) circuit. It has two halves. Setting contacts are in the negative feed and locking contacts in the positive feed.   At any one time only one Lock relay should be up corresponding to the position to which the points were last called. Unlike the route lock relays, both NLR and RLR are latched relays. There is no distinction in safety terms between the normal and reverse positions for points.

Except in special circumstances, points controlled from a route setting panel are not returned to the normal position after use.

3.2.1 Lock Relays 

The points normal lock relay (NLR) and points reverse lock relay (RLR), perform route and interlocking functions associated with the points, they also control their operation.

On operation of the control panel buttons to set a route, the RUR is energized, providing the interlocking is free. The RUR contacts then set all necessary point lock relays which in turn operate the points to line up the route. With point detection indicating that the points are in their correct position and providing that the track circuits concerned are clear the signal control relay energizes via contacts of the RUR and button normalizing relays.

These relays are magnetically latched and remain in their last operated position. Therefore, before picking up one relay, it is necessary to energize the release coil of the other. This is accomplished by wiring the negative side of each release coil to the negative side of the operating coil of the other lock relay. As each lock relay operating coil is wired through a back contact of the opposite lock relay, one lock relay is proved down before the other lock relay can energize.

Therefore, before a points lock relay can be energized to drive the points to the next position, the lock relay for the existing position is proved down ensuring that all routes which lead over the points in their present position are normal before the points can move.

In the positive leg of the points NLR and RLR is the interlocking function between the points, and signals which lead through the points. Route (track) locking over the points, including selective overlap (tracks which will allow the points to operate to the vacant overlap), and back contacts of the opposite points lock relay, and detector relay, to prove those functions de­ energized before the lock relay concerned will operate.

In the negative side of the NLR circuit are RUR contacts of all routes which will set the points normal in series with a contact of the points (C) R (Lever Centre Relay) and in the RLR circuit, RUR contacts of all routes which will set the points reverse, together with a point (C) R contact

An alternative path is also provided for use when the points are to be operated under lever control for both normal and reverse operation.

 3.2.2 Circuit Operation

When a call is placed on 101 points to operate reverse, e.g., 3M(A) route has been called, 3(M)A RUR will energize closing the negative leg for 101 NLR release coil and 101 RLR operating coil via the lever centre relay (C)R, to negative.

Providing the points are free to operate, i.e., 101 WZR (points free relay) is energized, indicating that the interlocking is correct, and the track circuits through the reverse route are clear, 101 NLR will be driven down via front contacts of 3M(B) & 3(S) B NLR, 3ATPPR and 3XTPR tracks (interlocking and track locking for the reverse route) 101 NLR and 101 WZR and WJR.

This action closes a back contact of 101 NLR in the positive leg of 101 RLR operating coil, proving the NLR has de-latched and allowing the RLR to energize (latch up). A front contact of 101 RLR now connects the WZR to 101 NLR circuit in readiness for the points when called to operate normal.

3.3 Locking Circuits

The positive feed to the NLR/RLR circuit checks the availability of the points to be moved, either normal to reverse or reverse to normal.

The feed to the WZR can be obtained from either the normal or reverse branch of the circuit, dependent only on the present state of the NLR and RLR. If WZR is able to energize it shows the points are free to move from their present position.

3.3.1 Point Free Relay (WZR) & Point Timer Relay (WJR)

The WZR or points free relay(Refer Figure 21)  is a slow to release relay to prevent the RUR from dropping out during the operation of the points lock relays. It taps off the interlocking and track locking portions of both NLR and RLR. When the NLR is energized the WZR detects if the points are free to move reverse. When RLR is energized the WZR detects if the points are free to move normal. Thus, the WZR relay when energized indicates if the points are free to operate to the next position.

The WZR is used to convey this information to the route RUR circuits which are allowed to energize if the required point lock relay is energized or if it is free to be energized.

A point timer relay (WJR) is provided to ensure that the tracks have been free for a length of time to cover "bobbing" tracks.

The WJR(Figure 21)  is a slow pick-up relay, which together with the slow pick-up track repeat relays provides a two-stage timing before the points are free. The WJR is tapped off the points lock relay circuit, and a contact of this relay cuts the WZR. The WJR is provided in the NLKPR and RLKPR circuits. (Figure 21).

A contact of the WZR is also used to illuminate the points free light above the centre of the lever and indicates to the signalperson when the points lever may be operated to drive the points to the next position. The only interlocking information not conveyed by the WZR relay is the point-to-point locking and this is added to the points free light circuit.

The WZR relay and WJR point timer relay in conjunction with the transient nature of the button controls provides for non-storage operation of the points under route setting conditions. If when the buttons are pushed to set up a route, the point lock relays are not in the correct position or free to be operated to that position as indicated by the WZR relays for the one second period during which the button relays are energized, it will be necessary to operate the buttons again when the route is free. If a train were passing over points within the route in question the security of the points is dependent entirely on the track relays remaining down whilst occupied by the train. Therefore, if the track relay should "bob" during the one second which the button relays are energized the points would commence to move under the train. To guard against this event, track repeat relays are made 4 seconds slow operating so that local tracks in the point circuit must be clear for 4 seconds before the points become free to operate to the next position.

3.3.2 Lock and Detector Repeat Relays (LKPR)

Refer Figure 21 .The circuits for these two relays which tap off the points lock relay circuits are the points Normal and Reverse lock and detector repeat relays (NLKPR and RLKPR). Contacts of these relays are used in the signal control circuits to provide proof that the detection and lock relays are in their correct position and that the operation of a route RUR has locked out the points lock relay for the movement to the next position before a signal can clear.

The NLKPR taps off the normal lock relay circuit so that it includes all interlocking which prevents the points from driving normal. In the case of 101 points, 3(M)"A" and 3(S)"A" NLR's, proving that routes which require 101 points reverse are normal. It ensures that 101 NLR and 101 NWKR are normal. It also ensures that 101 ROLR, 101 RLKPR, 101 WZR and 101 WJR are de-energized by back proving contacts.

The proving of 101 WZR de-energized is most important, and its function is as follows.

With 101 NLR energized (latched up), and 3(M)"A" route is called, the release coil of 3(M)"A" NLR is energized when the commence (CeR) and finish (FnR) button relays are operated and when 3(M)"A" NLR makes its back contact, 3(M)"A" RUR is energized. When 3(M)"A" NLR opens its front contact the circuit for 101 WZR and 101 NLR is opened and 101 WZR drop contact makes to allow 101 NLKPR to energize and complete No. 3 signal HR circuit.

Therefore before No. 3 signal can clear proof is obtained that 101 RLR circuit is opened via 101 WZR de-energized and therefore 101 points cannot be operated to the reverse position.

101 RLKPR taps off 101 RLR circuit and performs similar functions to 101 NLKPR, being utilized in signal control circuits which lead over 101 points in the reverse position.

3.4 Calling the points by route setting 

Energization of an RUR in the negative feed of the lock relay circuit will set the points provided the point key is central and the points are not locked by other routes, track locking or route locking.

Energizing the RUR will have proved that the points are in position or available (e.g., NLR or WZR up for a move to the normal position).

Contacts of the RURs for each route shown in the control tables to set the points will be wired in parallel in the NLR or RLR negative feed. Conversely the NLRs for the same route must be included in series in the positive feed of the opposite lock relay.

3.5 Moving the points using the point key/lever

Relays (N)R and (R)R repeat the panel key/lever in the normal and reverse positions respectively. They allow the points to be moved individually.

It is important to note, WZR must be up at the time of setting with the point key or the points will not move, even if any locking is later removed. Therefore, when moving the point key from normal to reverse (or vice versa), it must be held momentarily in the centre position to allow the WZR to reoperate.

3.6 Points in Overlaps

There are situations, generally where swinging overlaps are involved, in which the simple use of route RLRs to call the points is not sufficient.

Relays NOLR and/or ROLR may be provided to give the required point setting commands Refer Figure 22.

Overlap relays automatically set facing points in the overlap of a signal to give a clear overlap for that signal. When a route which has facing points in its overlap is set and the points are lying so that the overlap over which the signal would clear is occupied, but an alternate overlap is clear and the points are free to operate to that overlap, the overlap relay OLR will energize and drive the points to that position. The controlling signal for the route then clears via the free overlap.

The OLR relays are only energized during the one second period that the button relays are energized and thus comply with non-storage requirements.

Protection against the OLR's causing the points to move if a track relay should bob under a train is obtained by wiring a contact of the relative point WZR relay in the OLR circuit, thereby ensuring that the points have been free for at least four seconds before they can be operated to another position.

If the points in the overlap of a route are not free to move to an unoccupied overlap when the route setting buttons are operated, the route RUR will energize providing its requirements are met but the OLR will not be energized. Because of the transient nature of the button controls it will be necessary to either re-operate the buttons when the points become free or to set the points to the required position by operating the point lever.

3.7 Point Operation & Detection

The position of the NLR and RLR in the interlocking must be translated into a command to the points to move to or remain in the corresponding position. This is done by means of a polarized circuit to the NWR & RWR at the location. A typical circuit is shown on Figure 23.

Points control is affected through the NLR and RLR. Contacts of the relevant lock relay operate the normal points contactor (NWR) or reverse points contactor (RWR).

The points contactors can be of the type that are mechanically interlocked with each other and are installed in the points locations, or polarity sensitive relays installed in the points location or provided within the point machine, and which will only energize if the polarity of the supply to the coils is correct. Relays installed in the points location are type QBCA1 and have two heavy duty front contacts that are capable of switching about 10 amps current to the point motor.

Referring to the circuits above, each contactor is double switched by contacts of the points lock relay concerned. The points NLR when latched up will pick up the normal contactor and the points RLR when latched up, will pick up the reverse contactor. The opposing lock relay is proved down in the relevant contactor circuit in each case.

This provides a measure of safety so that if both lock relays should be up at the same time or if either lock relay is unplugged both contactor coils are open circuited.

When energized the motor will run either normal (if NWR energized) or reverse (if RWR energized). They are polarity sensitive relays, usually QBCA1 and will only energize if polarity i s correct. Positive to R1 and negative to R2 contact. These relays have two heavy duty front contact which are capable of switching about 10 amp current on and off to the point motor.

They are energized by the NLR energized and RLR de-energized in the case of the NWR, and the NLR de-energized and RLR energized as in the case of the RWR. These contacts are double cut into the point relays, ie contacts in both positive and negative feeds.

Each relay is also controlled by the opposite relay being de-energized, ie RWR proved down in NWR circuit and vice-versa.

There are two contacts of each relay in the opposite circuit, ie: referring to the circuit diagram RWR A6/A5 and D6/D5 are in the NWR circuit. This is to prove that the whole relay is de­ energized as it is possible to have half the relay 'stuck-up' by failure of a set of contact strips. Relay rows A and B are operated by one strip from the armature and relay rows C and D from another. This then proves that the RWR is definitely de-energized before the NWR can pick and vice-versa.

Once the points have moved to the correct position, a polarized detection feed comes back to energize NWKR or RWKR (Figure 24) provided the detection corresponds to the position required by the interlocking (NLR/RLR).

The detector relay circuits (NWKR & RWKR) as well proving that the points  have corresponded to the lever and are locked (via NKR or RKR), also proves the following:-

1. The opposing WKR de-energized, via a back contact 

2. The corresponding NLR or RLR energized, thereby ensuring all interlocking functions are correct 

3. The LWR (E.P. points,NOT SHOWN ) or Isolating Relay (electrical points) de-energized via back contact . This ensures that the points cannot be operated, except under normal operating conditions.

4. For EP. points (Not shown here) that the Plunger Lock has returned to the normal position (locked), via plunger lock normal contacts. This ensures mechanically that the points will not move should the control valve be falsely energized, or "creep" open due to worn equipment.

5. For electrically operated points, a contact of the EOL is included to ensure that while the EOL is withdrawn from the lock for emergency operation of points, both WKR's are de-energized, thereby ensuring that the signals protecting the points cannot be cleared, or the points operated from the control

With the points normal and called reverse, the points NLR is driven down and drops the normal detector (NWKR). A back contact of the NWKR closes the circuit for the points RLR, allowing the relay to latch up, providing that all interlocking functions are correct. This allows the points to then operate to the reverse position.

The point operation circuit has the additional protection of the IR (isolating relay).

Refer Figure 25. This proves all signals reading over the points normal, all direct locking tracks clear and no trains between the points and a protecting signal (unless moving away from the points.

The WTJR (where provided) disconnects the circuit to the point contactors if the points have been running too long. This will avoid damaging the point motor or clutch if an obstruction in the points prevents them completing their movement. 

IR's (for electrical operated points) or LWR’s (for E.P. points) prevent irregular operation of the points should the point lock relay, contactor or control valve be falsely energized whilst a train movement is taking place over the points.

The IR associated with electrically operated points can be of the neutral contactor type or a polarity sensitive relay and is installed at the points so as to be physically remote from the points contactor to prevent manipulation, or in the points location where the points contactors are of the relay type and thus sealed or located in the points machine.

The LWR is associated with E.P. points and is normally located adjacent to the points. When energized the LWR unlocks the facing point lock via the plunger lock and allows the points to move.

The IR or LWR check that the home signals protecting the points are normal and not approach locked, and that tracks from the home signals to the points, and the local tracks over the points are clear before the points can be operated to the next position. When the points have reached the required position the IR or LWR is open circuited by either a switch machine contact where the contactor type is used, or the relevant local detector relay (NKR/RKR) energizing where polarity sensitive relays are used and proved de-energized in the detector relay circuit (NWKR or RWKR).

Where polarity sensitive relays are used, for electrically operated points a contact of the EOL (Emergency Operating Lock) is provided and when operated manually, the isolating relay is open circuited.

The NKR and RKR (Normal and Reverse indicating relays), are located locally at the points and prove that the points have corresponded to the lever movement and are locked. They are divide into (2) two basic circuit types, those for E.P. points and those associated with electrical operated points. 

Figure 26 shows a typical NKR and RKR circuit used for electrically operated points using polarity sensitive relays. The points are proved Normal or Reverse and locked before the corresponding detector contacts are allowed to make. The opposing KR is also proved de­ energized via back contacts.

4 ROUTE LOCKING 

4.1 Principles

The control tables will often specify route locking to allow the route to be held in front of a train whilst being released section by section behind the train. This is effective as soon as the route is set and releases only after the passage of the train (or if no train has entered the route after the signal approach locking is released).

4.2 USR (Route Stick Relay) Circuits 

The relays used to lock each part of the route are called USRs, Route Stick Relays, which are energized when that section is free of route locking in the direction specified, and de-energized when route locked. A typical USR circuit is shown on Figure 27

 

The presence of the JR contact in the circuit will depend on whether the control tables specify a timed release.

The route stick relay in route control systems of interlocking performs a similar function to those in conventional interlockings where it may be used to:

• maintain or hold the route locking to provide maintenance of selective overlap.

• hold the route locking where a train has passed an outer protecting signal which is interlocked with the points, and the signal normalized with a train occupying the track circuits ahead of that signal.

• qualify that portion of the route locking that would not be required where the route is signalled for both directions.

The route stick relay is a normally energized relay with a stick function the relay being held energized by the signal concerned at Stop (ALSR Energized). The relay is de-energized when the signal is cleared and will remain de-energized with the track circuit ahead occupied although the route has been normalized.

The USR is dropped by the ALSR down (signal cleared) and proved de-energized in the signal HR circuit.

Under certain conditions the USR may be required to be timed out to release the locking, and where this is required a front contact of the track time limit concerned qualifies the stick function to allow the relay to energize at the completion of the timing cycle.

An example of the function of a USR relay is shown in Figure 21 where 1 USR is used to hold the points lock relay de-energized for maintenance of selective overlap. 

5         SIGNAL ASPECT CONTROLS 

 5.1     Aspect Requirements

 Once any route has been set, it must be proved entirely, including any overlap before displaying an appropriate proceed aspect and relevant route information to the driver.

This may include track circuits and/or points depending on the type and geography of the route.

 5.2 UCR Circuits 

The UCR proves continuously that all conditions are present for the signal to clear. A UCR will generally be provided for each route.

The UCR relays are mounted in the main location and include all the functions normally placed in the HR circuits. In effect the UCR is an internal HR relay. The HR relays are located in the remote locations. The UCR drops the NGPR and then the USR and ALSR relays which are proved down in the outgoing HR circuit, in series with front contacts of the UCR. The UCR relays allow proving of internal relays.

UCR circuits will generally contain the following controls: -

a) A contact of the relevant RUR which only operates when the route is required to set.

b) The SR, which allows the signal to clear for one movement only 

c) Track circuits proved clear by TPRs. For main routes, the tracks will be proved clear to the end of the overlap. Where facing points exist in the overlap, tracks beyond the facing points will have NWKR or RWKR contacts in parallel to exclude tracks when the points are set away from them .

d) Points set and locked, using NLKPR and RLKPR contacts 

Also shown in the circuit examples are back contacts of the TZR (this will be present when automatic nominalization is required) and down proving of any track circuit timers which will be used to release route locking associated with the route.

Refer Figure 28 for a Route Checking Relay which the UCR circuit for No.1 signal where the route is proved set by the RUR being energized thereby ensuring that all interlocking is correct and all relevant track circuits, including selective overlaps are proved clear (energized).

This Figure 28  shows the UCR circuit for No3 Signal has four routes    

• Main Route M(B)
• Shunt Route S(B)
• Main Route M(B)
• Shunt Route S(A)

5.3  Different Types of Routes 

Where controls are common between different types of routes (e.g., 3(M)A and 3(S)A), part of the circuit can be common to both UCRs. In the example shown on Figure 28, the main route UCR will include track circuits, but the shunt route will not.

Such circuits can often be laid out in a geographical manner. The circuit designer should decide the most efficient layout by reference to the signaling plan and control tables. Where main and shunt routes exist from the same signal, the track circuits and overlap points will have to be separated out to appear in the (M)UCR circuit only.

5.4 Stick Relay 

The function of this relay is to maintain the signal at stop after the train has passed it. The signal will clear for one train movement only.  Once the train has occupied the first track in the route, the stick relay can only be reoperated by normalizing and resetting the route. If automatic working is required, an (A)SR will be provided to maintain the SR circuit energized.

The lever stick relay (SR) performs the same function as in a conventional interlocking. When a train passes a signal, the signalperson must pull the panel button to normalize the route before the signal can be cleared again.

Referring to the circuit Figure 29, with the passage of a train passed No 1 Signal 1, SR is de­ energized by 1AT track dropping and will remain down after the train has vacated the track until No 1 panel button is pulled to energize 1(N)R relay, where a pickup circuit is established via 1AT and 1(N)R contacts. 1 SR is held energized via 1AT track contact and 1 SR stick contact when the route is set by the operation of the panel button.

A front contact of 1 SR is included in No 1 signal control circuit (1 HR or 1 UCR if provided) and after the passage of a train past No 1 signal the SR contact prevents the signal from clearing again until the route is normalized and then re-set by the operation of the panel buttons.

The (N)R contacts in parallel with the route NLR contacts allows re-energization of the SR relay should power failure occur when a train is approaching the signal and the signal is showing a proceed indication, under which conditions the approach stick down would prevent energization of the route NLR (approach locking) when the panel button was pulled and it would not be possible to energize the SR relay to re-clear the signal unless the timing period of the ALSR had elapsed.

5.5 Approach Lock Stick Relay (ALSR)

5.5.1 Approach Locking (Requirement)

Approach Locking is achieved by means of an Approach Stick Relay (ALSR) and is provided on all controlled signals with the exception of certain starting signals. Its purpose is to hold the route locked, thus preventing the operation of points in the route and/or the setting of a conflicting route if the signal protecting the route has been returned to stop in the face of an approaching train.

A route becomes approach locked once a driver has seen a 'proceed' indication or has seen an indication at a previous signal which would indicate to him that the next signal is displaying a 'proceed' indication. Where long sighting distances are involved, 600 meters is considered a suitable approach locking distance to the first warning signal.

The approach stick relay is energized by front contracts of the NGPR i.e. signal at stop, and the approach track or tracks circuits to that signal unoccupied and will remain energized with the signal at stop via the stick path with the approach track occupied. The relay is de-energized when the signal for the route is cleared and will remain de-energized with the approach track occupied although the signal has been normalized. A front contact of the approach stick relay is included in the route NLR and prevents this relay from normalizing (latching up) when approach locking occurs as described above, thus preventing release of the interlocking.

When a route becomes approach locked it is impractical to hold the route locked indefinitely once the train has come to a stand. To overcome this and the need for the signal electrician to provide a 'release', a time release relay is provided (ALSJR). The relay commences its timing cycles once the signal has been returned to the stop position (NGPR) energized. A timing cycle of 120 seconds is provided for main line running signals and is considered sufficient to ensure the train has come to a stand. For shunting signals a time limit of 60" is provided.

A front contact of the time release relay is placed around the stick function of the ALSR and when energized allows the ALSR to energize.

5.5.2 Approach Locking (Operation)

The circuit for 3 ALSR as shown on Figure 30,and its various circuit paths are as follows: PATH No 1:- 3 ALSR will energize (approach locking not effective) if No 3 signal is at stop, (NGPR energized) and track circuits approaching No 3 signal (1AT and 1BT) energized, with the approach track to No 1 signal (54.5B) included if No 1 signal has not normalized (1 ALSR down). This arrangement satisfies the condition where a driver has seen an aspect at a previous signal which would indicate to him the next signal is displaying a proceed aspect.

The two-track occupation to release approach locking under normal running conditions is to overcome the problem of a track bobbing under a train thus releasing the locking.

PATH No 2:- Allows for energization of 3 ALSR when a train proceeds past No 3 signal in the normal manner and allows a release of approach locking should a long train be standing with its rear on the approach locking tracks. To guard against a release due to an intermittent failure of 3AT, either 3BT or 3XT must be shunted at the same time. To guard against a premature release due to a power failure and restoration, which will cause the track circuit PR’s to drop and then pick up, a front contact of POJPR, a power off time delay relay, is included in the release path. The POJR, which is the parent relay for the POJPR, is wired directly across the AC supply and does not make. Its front contacts until 30 seconds after the supply is restored.

PATH No 3:- The stick circuit holds the ALSR energized with the protecting signal (No 3) at stop and a train occupying the approach tracks.

PATH No 4:- Energizes the time release which allows the release of the approach locking when the signal has been cleared and then returned to stop with a train occupying the approach tracks.

5.5.3 Approach Locking (Testing ) 

There are 4 approach locking test performed in the logic.

Test1: Did the signal always stay at stop ?That is,  signal not shown a proceed aspect .Then it is safe to normalise the route when controller cancels the route  ,test passes and route get cancelled straight away

Test fails if signal started to clear or made an attempt to show a route indicator and if lamp is failed ,driver believe a proceed aspect.

Test 2 : Has no Train approached the signal ?Its is permitted to cancel (normalise) ,if approach tracks are clear (ie .Upto sighting distance of first caution-Comprehensive Approach locking ) .This means if no train approached test passes and route  get cancelled .If Train has approached the signal and controller put signal to normal ,test fails .This is checked for the track status from concerned signal looking back to first caution aspect (Comprehensive approach locking ) .Test 2 is required only when route is not approach controlled for Red (MAR ) or an automatic working facility.

Test 3:-Did Train enter the route ?If signaller cancels the route when train  entered the route already ,test passes because sectional route locking will protect the train (USR relay above ) .Test fails when route is cancelled while train occupy birth track and just entered .Sectional operation of track is 1st track clear ,2nd track  occupied ,AFTER first track occupied and second Occupied 

Test 4 : Did Train get Time to stop?.This is the last resort to normalise the route ,if all above 3 test fails when test sequence started and train seen a proceed or impression of proceed aspect .Then timer operates for the train to come to standstill OR  have to wait for train to pass the signal for sectional route locking to release (USR) .In UK practice for a Main Signal Timer is 180 seconds for comprehensive approach locking and 120seconds for Signals get approach locking "when cleared"   and in Australia it allow only 120 seconds for a train to come to standstill for main routes  or 60 seconds for shunt route  and approach locking get released after timer times out (Timer path on the ALSR relay ) .In nutshell for main line or loop line timer value is based on the class of route (Main Route  /Diverging Main Route) ,which is 180 seconds for UK and 120 seconds in NSW ,Australia.

If the route is Call on (Position Shunt Route  ) ,irrespective of straight route or shunt route approach release timer value will be lesser .Check with your railway for these values !

5.6  Signal Operation 

The UCR is at the interlocking. This will be used to operate a circuit to the HR at the location, controlling the clearance of the signal. Refer Figure 31 for a typical HR circuit for 1 Signal and 3 Signal where 3 Signal have both Main and Shunt routes.

The HR (Signal Control) Relay operates the signal lights to show a proceed indication from the Stop position and is located at the signal location.

The NGPR which proves the signal and train stop, if provided, have returned to normal, is proved de-energized in the HR circuit via back contacts.

The ALSR and ALSJR are proved de-energized and ensures that the approach locking requirement is effective.

The USR where provided is back proved thereby ensuring that the route locking is effective.

The UCR proves that all points are detected in the correct position and that the track circuits are unoccupied for the route set.

Refer Figure 32 for a Typical Signal Control Relay (DR) for green aspect.

The DR Relay when energized provides the full clear (green) indication in the signal. The relay is energized by a front contact of its own HR and the HR for the signal in advance.

Where single light colour light signaling is used, a front contact of the ECR for the signal in advance is included. This ensures that if a lamp fails in the signal in advance, the signal will only display a caution indication.

The VRR of the signal in advance is also included when train stops are provided.

To prove that the signal has responded to the interlocking control, an NGPR (signal normal - at stop) and an RGKR (signal cleared) are fed from the signal location back to the interlocking.

The NGPR (Normal Signal Repeat Relay) proves that all signal control and operating functions, ie:- UCR's HR's and train stop if provided, have returned to the normal position, signal showing stop indication.

The NGPR conveys this information via a front contact to the ALSR for the necessary proving, and the stop indication for the signal repeater in the diagram. It is also proved de-energized in the signal HR circuit.

The (RGKR) Reverse Signal Indicating Relay, indicates that the signal has been cleared and is energized by front contacts of the HR relay concerned. This relay provides the clear indication for the signal repeater. Refer Figure 33 for Normal /Reverse Signal Repeat Relays

Proof of signal normal is vital in the approach lock release. Both relays are used to provide control panel indications.

 

 6 ROUTE RELEASING

 6.1        Principles 

Route releasing comprises the following sequence of events: -

A) Initialization of route release - signalman pulls the signal button at the start of the route .If automatic normalization is provided, this will be initiated by the train passing the signal.

B) Release of approach locking on the signal - the train is proved past the signal (this may occur before  or after (A)), the train  has come to a stand  at the signal or there is no train approaching.

C) Release of the route up to the rear of the train, if any, known as sectional route release.

6.2 Approach Lock Release 

As far as the circuits are concerned, the first step must always be to operate the ALSR. The circuit is shown on Figure 30 above. Three separate circuit paths are provided to pick the ALSR according to whether the train is entering the route, the train has come to a stand at the signal or there is no train approaching.

The arrangement of this circuit will be determined by the "approach lock tracks" and "approach locking released by" sections of the control table.

Once operated, the ALSR will remain up until the signal is ready to clear again  for another movement.

6.3 Route Release 

Picking the ALSR will allow the NLR to latch up (Refer Section 2.12) provided the route has actually been cancelled.  This, in turn, will allow the USR (or the first USR if there is more than one) to pick as soon as the tracks are clear. It can therefore be seen that the route locking will always release behind the train.

If the route is cancelled with no train, the USRs will pick up immediately as all the tracks are clear.

7. OVERLAPS 

7.1 Principles

 Interlocking circuits are considerably complicated by overlaps. The controls must be accurately specified in the control tables and then translated into additional calling and locking circuits.

The circuits must ensure that any route requiring an overlap has that overlap (or an acceptable alternative) maintained as long as the route is set or there is a train in the route.

7.2 Aspect Circuits 

The UCR circuit will include the additional point detection and track circuits in the overlap. For signals with facing points in the overlap, all valid overlaps will be included, with the necessary conditions of the position of the facing points. The facing points themselves will only be detected where they are set and locked to prevent a confliction with another route.

7.3 Route Locking of the Overlap

 Where an overlap is provided, route locking must extend to the overlap. Normally a timed release is necessary to prove the train has come to a stand and allow the overlap to release when no route has been set forward from the next signal.

The USR will then include a TJR contact for the last track in the route in parallel with the TPR front contacts.

8 PRESET SHUNT SIGNALS 

Occasionally a preset (or facing) shunt signal is positioned within another route. It may either be operated on its own as a shunt signal or cleared by setting the main route (presetting). Once a train has entered the main route, the preset shunt must remain off until the train has passed it. The preset shunt may be replaced to danger in emergency, but this will not permit any release of the main route beyond the preset shunt.

The examples do not include a preset shunt signal. The main points to note are as follows: -

a) Main routes which require the shunt signal to be preset will first prove that the corresponding routes from the shunt signal are not in use (and vice versa).

b) Setting of the main route will initiate the presetting process.

c) The main signal will prove the shunt signal cleared in its UCR circuit. Pulling the signal button of the preset shunt will therefore return the main signal to stop.

d) Route locking for the main route will be effective to the end of the route, not just to the preset shunt signal. Once the train has entered the main route, it cannot be partially released beyond the preset shunt . A train cannot therefore be re-routed by cancelling the preset route and resetting for another route.