Automatic Train Control is the CBTC system which automatically controlling the train movement while enforcing safety. ATC provide command to motoring the train, coasting, braking, regulating the speed with accurate station stopping. This system automatically protects the train (ATP) while ensuring safe separation between trains and protection from over speeding. Refer Fig 1 for the functionality of Automatic Train Control. Through out this document “Trackside”  means equipemnt installed on track or along the geometry of the track , “Wayside”  Means  equipment associated with Signalling Equipment Room /Control Room and Trainborne means equipment installed on the train.

Fig 1  Functionality of Automatic Train Control

Based on Functional Elements, ATC has wayside equipment and Train borne equipment to perform this functionality.

a)    Train Borne (Signalling) 

• • Train-Borne Signalling Cabinet ( ATO , ATP ,TDM )
  • Radio Antennas
  • Under Train Sensors (Balise Scanner)
  • Tachometer
  • Video Display Unit (VDU)

b)  Train Borne (Train Supplier)

• • Speedo Meter and Or Odometer
  • Brake Relay Encoder Cabinet
  • Train Integration Management Cabinet

c)  Trackside 
     

• • Balise
  • DCS Access Point (AP) for Wifi

d) Wayside

• • DCS Equipment Cabinet
  • Wayside Computers for ATP and ATO (Zone Controller )
  • Interlocking Subsystem
  • Automatic Train Supervision Subsystem 

Functionalilty of Element

1.1    Train-Borne Signalling Cabinet – Train Borne ATP System 
Automatic Train Protection processing unit belong to the train borne computer system.
This subsystem is in charge of the continuous control of the train speed according to the safety profile and applying the brake if it is necessary. It is also in charge of the communication with the wayside ATP subsystem in order to exchange the information needed for a safe operation sending speed and braking distance, and receiving the limit of movement authority for a safe operation. Refer Section 1.15 for wayside system details 

1.2    Train-Borne Signalling Cabinet – Train Borne ATO System
Automatic Train Operating Processing unit belong to the train borne computer system, 
It is responsible for the automatic control of the traction and braking effort in order to keep the train under the threshold established by the ATP subsystem. Its main task is either to facilitate the driver or attendant functions, or even to operate the train in a fully automatic mode while maintaining the traffic regulation targets and passenger comfort. It also allows the selection of different automatic driving strategies to adapt the runtime or even reduce the power consumption. Refer section 1.14 for wayside ATO system details

1.3    Train-Borne Signalling Cabinet-TDM
Train Data Management Processing System is providing the interfacing functionality with Trains Integrated Management. This is the processor-based System offer health monitoring, train event recording and control of train initialisation. It is also the interface for train door control, command train to wake up and sleep. Processor process the data received from various train borne subsystem and send to the ATS. 

1.4 Radio Antennas 
Antennas are generally installed on exterior roof of the train, part of the Data communication system Transmit and Receives (Bi -Directional ) processed data between Train-Borne  Radio Unit and Trackside Data Radio Unit . Refer Access Point section 1.12  & DCS 1.13

1.5 Under Train Sensors (Balise Antenna ) 
Part of Geographical Position (Location Reference ) functionality of a CBTC solution  ,train borne balise transmission module radiates energy wave  to activate the ground balise to uplink geographical position  transmission  and these sensors are mean  for collecting the spot transmission data .
Spot transmission wayside devices (Fixed Balise) provide the train with information allowing the train to check and to calibrate its odometer, and to identify the actual train location. In general on CBTC solution  rest of the bi directional data transfer is  happening through radio.
Refer  Balise section  1.11

1.6 Tachometer 
This is the speed sensing device otherwise known as wheel impulse generators or speed probe .It can be opto isolator slotted or Hall Effect sensors .It detects the speed of the train and passes on to Train-Borne controllers(ATP and ATO)   and speed measured b Tachometer  is also used to ensure train at stand still before TDM inform TIM to open the door. Speed will be displayed on the speedometer

1.7  Video Display Unit (VDU) 
Optional unit for GoA4 (Grade of Automation 4), however most of the CBTC has a VDU , accessible only when needed ,especially during  the degraded mode of Train Operation. Its kept covered in a box and accessible for Train Operator when needed.  GoA1 and GoA2 will necessarily have a unit displaying speed ,brake enforcement needs ,warning etc (Train Protection ,Train Control Information ) ,mode of operation  selector switch , driving ontrol handle  etc where as GoA4 VDU could be displaying equipment operating staus ,failure conditions rest are  defined based on operator needs. Refer Fig 2 for a ruggedized VDU designed per EN 50155 / EN50121-3-2 / EN61373.
 

Fig 2  Video Display Unit       

1.8    Speedo Meter
Unit which display of the speed of the train 

1.9    Brake/ Relay Encoder 
The Trainset unit automatically control the speed and regulate the speed based on the information received from  ATP and ATO processors.

1.10    Train Integration Management (TIM)  
The system which interface between Signalling and Trainset  ,which provides health monitoring status to Train Data Management(TDM)  upon request .When ATS issue Train readiness command  via TDM  to TIM and ATC for their preparedeness for readiness .Train(TIM)  and ATC set ready and send the readiness status back to the  ATS via TDM .If readiness is not available a fault code will be send back to ATS .TIM use the clock along with TDM to ensure synchronisation.TIM also communicate with the Passenger Information System with the same “clock”

1.11    Balise 
The Track Installed Transmission System performing a safe spot transmission, conveying safety related information between the track and the train. Information transmitted from an Up-link Balise to the On-board Transmission Equipment is fixed Spot transmission, when a transmission path exists between the wayside equipment and the On-board Transmission Equipment at discrete locations. The information is provided to the train only as the Antenna Unit passes or stands over the corresponding Balise. The length of track on which the information is passed is limited to approximately one meter per Balise.For CBTC application fixed balises are widely used to provide the train with information allowing the train to check and to calibrate its odometer, and to identify the actual train location. In nutshell in a  CBTC  solution  rest of the vital  bi directional data transfer is  happening through radio. Refer  Balise Antenna  section 1.5.

1.12 Access Point (AP) of the Data Communication System ( WiFi ) 
An access point is a wireless network device that acts as a portal for devices to connect between Wayside and Train borne Equipment installed along the Trackside. Access points are used for extending the wireless coverage of a wired DCS network so that the train passes by the area covered by an access point can establish seamless bidirectional data transfer. A high-speed Fibre Cable a Data Communication Cabinet with router from the equipment room to an access point, which transforms the wired signal into a wireless one. Refer Radio Antennas section 1.4 & Data Communication System 1.13 which work hand in hand.

1.13 Data Communication System (DCS ) 
The Communication network formed by redundant fibre optic cables based on geographical layout of the railway. This is making use of cable route diversity to ensure no single point failure Occurs. DCS network is normally formed in a ring topology so that any components fails availability is ensured by re routing the communication within the ring. Dual switches are provided at each location for the availability of the local area network.

1.14 Wayside Computers -Wayside ATO System.
The system in charge of controlling the destination and regulation targets of every train. The wayside ATO functionality provides all the trains in the system with their destination as well as with other data such as the dwell time in the stations. Additionally, it may also perform auxiliary and non-safety related tasks including for instance alarm/event communication and management, or handling skip/hold station commands. Refer section 1.2 for Train borne ATO system details.

1.15  Wayside Computers -Wayside ATP System 
This subsystem undertakes the management of all the communications with the trains in its area. Additionally, it calculates the limits of movement authority that every train must respect while operating in the mentioned area. This task is therefore critical for the operation safety. There are bidirectional communication established between wayside and Train borne ATP system Refer section 1.1 for Train Borne ATP system.

1.16 Interlocking system 
The system  which control the wayside equipment such as point machine ,signals (for fall back mode) , and gathers the secondary train detection system such as track circuits /axle counters (train recovery ) for locking the wayside equipment in front of a running train .Depends on utilisation of  the non-core functionality of  CBTC trackside equipment vary .Interlocking will become master during degraded mode ,during a complete ATO failure ,for recovering the train ,again depends on operator definition.

1.17 Automatic Train Supervision System (ATS) 
The system responsible for controlling and monitoring the unmanned train operation according to time table or headway .There is another article detailing Automatic Train Supervision in RailFactor .
Feel free to refer to that for more details .
Architecture of  ATC (CBTC ) 
Refer Fig 3 for System Architecture of an Automatic Train Control System(Distributed) 
 

Fig 3  CBTC System Architecture

ATC constitute of the complete subsystem as shown in the figure 3. It varies from supplier to supplier. Some of them have centralized architecture, and some have distributed architecture. In the figure 3, above shows a distributed architecture. For the explanation purpose consider .Three (3)   trains stabilized in the depot  are  to be operated with  3 min headway. 

2.    CBTC Operation 
For the sake of explanation, Automatic Operation (Un manned Train Operation) has been taken into consideration and degraded mode of operation or manual route setting ,or operator intervention sequence are not included .Sequence of  route call will be same as automatic ,only difference  is  that these process of route call is made manual from control centre ,or back up control centre or station control centre.

2.1.    Role of ATS
Primary objective of Automatic train Supervision is to control and monitor the train operation and manage the train operation according to timetable or specific headway.
Trains stabilized in the depot has a profile that can be uniquely identified by the ATS for tracking. ATS maintain a record of each train consist profile with a train identification.
Note : The train identification is a static field uniquely sourced from train supplier  with physical car numbers ,ATC number of the train. Refer section 1 .17 and another Article Automatic Train Supervision in RailFactor 

2.2.    Role of ATC (ATO/ATP/TDM) 
ATC receive the wake up command from ATS through the DCS network transmitted to the  trackside via Access point  .Train antenna mounted on roof of the train captures this signal and pass on to the train-borne ATC equipment .ATC has three functionality which includes ATO ,ATP and TDM .Refer section 1 for details of individual system within Train borne  ,Trackside and  Wayside (ATO, ATP ,TDM ,Antenna ,DCS ,Balise ) 

2.3.    Role of Interlocking
Interlocking is an arrangement of signalling appliances in which operation of one appliance will depend on the status of other in a proper sequence to ensure safe conditions . In other words  Interlocking is a failsafe system  responsible for  controlling ,gathering vital equipment  status , locking and releasing  trackside equipments such as Signals ,Points and other equipment .There could be various other trackside equipment   such as stop Plungers , track circuits ,and wayside equipments such as  control panel for degraded operation .These are part of the non-core functionality of the CBTC  which varies from operator to operator for train recovery ,maintainers protection, diagnostics ,fall back mode etc   .There will be a dedicated interlocking chapter published in RailFactor  in future.

2.1.1    Launching of Train from Depot to Mainline 
Based on time table , ATS automatically set a route for the train consists 1   assigned with a head code ,entered by the operator so as to allow the trains to reach its destination  without having the operator to set route manually.ATS will send the route request in advance to the interlocking to set the route before the departure time .Once the route is set ATS will remotely send the wake up command to switch Train 1 On .Upon receipt of this command on ATC computer (ATP/ATO) of Train 1  perform a wake -up test automatically. This wake-up test will test the operational and safety capabilities of the train-borne ATC(ATO /ATP/TDM)  system and its interfaces with the train and other subsystems .TDM will provide the front end processing for interface between ATC and Train Integrated Management system of the train. This is to allow data transmission between TIM of Train and ATC. As part of the wake-up test ,ATP system confirm that the train 1 has not moved during sleep and train position is known. Wake up sequence commence with door closed. In case door is open, then TDM (Train Data Management) will request door closed and check doors are closed prior to commencing the wake up sequence .In case door failed to close  an Alarm will be send to the ATS .Train will verify its geographical position through the balises  and its transferred to the  Train Borne equipment. When train is ready to start his ride ,ATP will send a signal to the relay /brake encoder panel of the train to release brake and train  1  takes the  route assigned to it and commences the journey  from depot to the mainline reception track  .Train 2 follow the same process following the initiation from ATS based on  Time table and enters into the main line reception track from depot following Train 1  (maintaining a headway of 3 min) . Train 3 will follow Train 2 maintaining headway of 3 min with Train 2. As shown in Fig 4, Train 2 will maintain a safety margin with train 1. This safety margin is based on various parameters such as Train 2’s  braking characteristics ,gradient of the track etc. In nutshell when Train 1 stops on platform 1, Train 2 will apply service brake to maintain safety margin by either reducing speed or a complete halt, until he can maintain safety margin with Train 1. All the trains are updating the geographical position and speed to the wayside  ATC system and receives a movement Authority continuously .Thanks to  the bi-directional ,radio system making continuous communication  between wayside to trackside(wired)  and trackside to the train (wireless).When Train passes  over the balises ,it update the position and send to the wayside ATC 

 
Fig 4 ATC Train Operation

 
3. Summary   
This article  covers the general operation using the core functionality of a CBTC system ,non core functionality  such as monitoring ,maintenance ,train recovery ,cut over etc are subject to operators requirement and are out of scope for this article.This will be covered in another article