Train Control Systems could be simply called as ‘Signalling’ is key to run the railways safely and reliable. As the train is a steel wheel on steel rail vehicle, stopping the train becomes harder task. It’s easy to get the train moving but much more difficult to stop in the right place and, to do this consistently and keep time, requires skill and concentration.

There are two basic reasons the railways need signalling system,

  • Trains are guided by the track and hence they need to be managed and routes to avoid collision with other trains.
  • Trains cannot stop within the visible distance of the driver , therefore they need early warning to slow down and eventually to halt.

Many train control systems have been developed and introduced to date to manage modern complex railways. Train control systems are essential for smooth traffic, thanks to real-time monitoring and reliable communication channels.


Present Railways have few named Train Control Systems,

Conventional: Signalling: Main concept is Fixed Block system

CABS: Cab based Signalling System

CBTC: Communication based Signalling system

ATC: Automated Train Control System can be implemented by CBTC and other systems

Each signalling evolvement is been discussed in detail below

Conventional Signalling


Main features of Conventional Signalling is

  • Drivers use the trackside signals to determine the if the train can proceed the journey and the speed the train will travel.
  • The track side signal will have three aspect signals (Red- Stop , Yellow-Proceed to next sginal with caution , Green – Continue to move)
  • Location of the train is determined by occupancy of track circuits.
    The Signal Controller will see the track as small lengths called ‘blocks’. Length of the block is determined by distance of the trains to be kept apart and how many trains are allowed from one block to other.
  • Smaller block lengths will increase the capacity of the line but reduce the margin of safety while increasing the length means more safety but less capacity.

Basic Concept: Allow one block separation between the trains to allow the safe operation. If the front train hits emergency break comes to stop then the following train will require the stopping distance to come to halt which will be effectively be the minimum length of the block.



CABS Signalling

Main features of CABS signalling is

  • Track Side signals are optionally used and usually not used
  • Track Circuits are used to determine the train location
  • The Speed and distance to continue the journey are displayed on the Train Overview Display (TOD) inside the cab.
  • This further enforced by ‘Automated Train Protection’ (ATP) system
  • CABS can allow the train at multiple speeds within a single block

CABS Signalling does not mean it needs to alter or remove the fixed conventional signalling concept. CABS can allow the trains to go at maximum speed even within smaller length blocks and reduce the headway between the trains  which effectively increase the capacity.



CBTC Signalling


Communication Based Train Control(CBTC) is an automated control system that ensures safe operation of rail vehicles using data communication between control entities. CBTC systems are modern railway signaling systems that can mainly be used in urban railway lines (either light or heavy)

Main features of CBTC signalling is “Track Side signals are not used and track circuits are not used”

Position of the train is determined by a 2-way communication between the train and wayside. Train transmits the position and wayside determines the target. This is effectively “moving blocks” system.


It also enables the wayside equipment to define the points on the line that must never be passed by the other trains on the same track. These points are communicated to make the trains automatically and continuously adjust their speed while maintaining the safety and comfort (jerk) requirements.


Automated Train Control (ATC)

Modern CBTC systems allow different levels of automation or Grades of Automation. In fact, CBTC is not a synonym for “driverless” or “automated trains” although it is considered as a basic technology for this purpose. The ATC system automatically controls train movement, enforcing train safety and directing train operations mainly via its two sub-systems, the Automatic Train Protection (ATP) and Automatic Train Operation (ATO) and additional sub-system called Automated Train Supervision (ATS) is also found in some railways.

Automatic Train Protection: The ATP subsystem maintains protection against collisions, excessive speed, and other hazardous conditions by combining the following procedures: train detection, train separation and end of authority protection.

The ATP ensures safe train separation by using the ATP track circuit status and by location determination. In order to verify that the train is able to stop at required stopping point to protect the train ahead, the ATP monitors the speed of the train, keeping it to an allowable speed. In the event of over speed, the train will initiate emergency braking to protect the train ahead.

Automatic Train Operation: The ATO system drives the train automatically to achieve prescribed operational performance within the safety constraints imposed by ATP. The main function of the ATO is to drive the train, provide accurate stopping position at the station and to control the train and platform screen doors.

Automatic Train Supervision: ATS supervises the overall operation of the train service according to a prescribed timetable or train interval by de-centralised processing through a network of distributed computers involving the automation of train supervision, with the flexibility for manual intervention.

Functions of the ATS:

  • Generate timetables to operate train services automatically
  • Obtain train positions and speed information from ATP system
  • Obtain train arrivals and departures from the ATO system
  • Automatically issue routing commands for each train according to timetable and train position
  • Adjust train timings by sending to ATO the dwell time at station and speed to the next station allowable by ATP (motoring and coasting data)
  • Facilitate remote central control of train service at the control centre
  • Facilitate the regulation of train services during train service disruptions (centrally-controlled at control centre)
  • Passenger information (schedules, arrivals and departures, destination)
  • Train service data and events (faults, alarms, train delays, deviation reports)




Levels of Automation

Automated Train Control (ATC) is another CBTC but different levels of automation which are called grades of automation available range as following


  1. Manually  protected operation, (usually applied as a fall back operation mode)
  2. Semi-automated Operation Mode, STO
  3. Driverless (Driverless Train Operation, DTO).
  4. Fully automated operation, (Unattended Train Operation, UTO).
  5. The latter operates without a driver in the cabin, but requires an attendant to face degraded modes of operation as well as guide the passengers in the case of emergencies.

The International Association of Public Transport (UITP) summarises automation of rail transport into five Grades of Automation:


There are huge benefits of CBTC,

Train Speeds can be optimised which would increase the line capacity at less cost and passenger comfort

Implementation of the system is usually short term and can be put in operation straight away.

Easy to migrate from transitional system as this can be implemented in migration zones.

Since backbones are fibre and way side wireless. , it provides  good immune system against many interfaces.


The typical architecture of a modern CBTC system comprises the following main subsystems:

Way Side equipment : the interlocking and the subsystems controlling every zone in the line or network (typically containing the wayside ATP and ATO functionalities)

On board equipment: CBTC on-board equipment, including ATP and ATO subsystems in the vehicles.

Radio communication sub system : Train to wayside communication subsystem, currently based on radio links.


CBTC architecture can vary from supplier to supplier but generally following can be found in the system


  • On board ATP System : In charge of the continuous control of the train speed according to the safety profile, applying the brake if it is necessary and communication with the wayside ATP subsystem
  • On board ATO system:  Controls the automatic control of the traction and braking effort in order to keep the train under the threshold established by the ATP subsystem.
  • Wayside ATP system: This subsystem undertakes the management of all the communications with the trains in its area and calculates the limits of movement.
  • Wayside ATO system: It is in charge of controlling the destination and regulation targets of every train.
  • Communication system: Integrate a digital networked radio system by means of antennas or leaky feeder cable for the bi-directional communication between the track equipment and the trains
  • ATS system: Act as the interface between the operator and the system, managing the traffic according to the specific regulation criteria.
  • Interlocking system: It will be in charge of the vital control of the trackside objects such as switches or signals, as well as other related functionality


European Railway Traffic Management System (ERTMS)

ERTMS is covered in seperateate article which can be found here

ERTMS level 3 is equivalent to CBTC for main lines.

ERTMS – Basic Guide to Europe Railway System


Available Control Systems in the Industry


Thales CBTC

  • Standard network and radio based communication system based on Internet Protocol (IP).
  • Trains can be monitored remotely in a control room in real time.
  • Constant fibre optic based high-speed connectivity between train and wayside
  • Detection of train location , speed and direction is performed by Transponder Tags , accelerometer and Switch/route information


  • Train separation is controlled by ‘progressive movement authority’ and trains are handed over between zone controllers
  • Wired backbone network net work that provides IP network services between the central or wayside equipment. The wired network consists of fibre optic Ethernet rings which allow high speed channels to each connected node.
  • Way side network utilises the backbone to connect to Wayside Radio Unit (WRU) to the overall network. WRUs are connected in a ring topology so if a single WRU fails all unaffected WRUs remain connected.
  • Each On board network device is with wireless interface and connected via single twisted pair wire.



About the Author :

Sujay Sujatharan

Sujay is an experienced Railway Engineer. He worked in Major Railway projects in UK including Crossrail , London Underground and Great Western Modernisation. Sujay has good experience in Integrated Railway Design and Interface Management.

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