Railway Signalling






Railway Signalling is a complex and fascinating research and development area in railways. The purpose of a signalling system is to facilitate the safe and efficient movement of trains on the railway. Two major worldwide markets in railways are Main Line Railways and Urban Rail Transportation Systems (Metro, LRT (Light Rail Transit), Tramway). This project investigates all aspects of railway signalling systems for İstanbul Urban Rail Systems to design and develop national signalling solutions. The initial target of the project is to build the research and development infrastructure (signalling literature and information infrastructure, railway signalling lab, development team, team processes and dynamics, ...)  and to produce the first road map, some technical blue prints, requirements analysis and system architecture design documents, and railway signalling simulator programs.

Fixed Block Systems

Looking back over the past few decades, railway signaling technology has been based mainly on the so called “Conventional Fixed Block System” (ref1, ref2) principle. Traditional signalling systems are based on fixed blocks: the railway is divided into sections of track, which are separated by signals. A train is not allowed to enter a given track section (=block) before the preceding train has cleared it. This system has a number of disadvantages, one being its lack of flexibility: the block size is the same for all trains regardless of their speed and braking performance. Thus the big safety distances required by fast trains are imposed on slower trains as well. Obviously this reduces track capacity.

The fixed block technology inherently imposed a service limitation because of the need to reserve buffer block(s) for train separation. With increasing patronage, demand grew to achieve higher line capacities on existing rail infrastructures. In order to realize this requirement without major upgrades to the rolling stock and rail infrastructure, intelligent signaling and train control systems have become a crucial technology for the new age of rail systems and services. The distance-to-go principle has therefore been developed on the “Fixed Block System,” which provides flexible control of the buffer block(s) for train separation.

Further to that, the “Moving Block System”, which also operates on the distance-to-go principle, has evolved. Moving block systems require less wayside equipment than fixed block systems. They provide considerable cost reductions for personnel and maintenance due to a strong reduction in way-side equipment.

Moving Block Systems (CBTC = Communications Based Train Control)

A moving block system (often called CBTC = Communications Based Train Control) does not require traditional fixed-block track circuits for determining train position. Instead, it relies on continuous two-way digital communication between each controlled train and a wayside control centre. On a moving block equipped railway, the line is usually divided into areas or regions, each area under the control of a computer and each with its own radio transmission system. Each train transmits its identity, location, direction and speed to the area computer which makes the necessary calculations for safe train separation and transmits this to the following train. The radio link between each train and the area computer is continuous so the computer knows the location of all the trains in its area all the time. It transmits to each train the location of the train in front and gives it a braking curve to enable it to stop before it reaches that train. In effect, it is a dynamic distance-to-go system. As long as each train is travelling at the same speed as the one in front and they all have the same braking capabilities, they can, in theory, run as close together as a few metres (e.g. about 50 metres at 50 km/h). This, of course, would contradict the railways safety policies. Instead, one safety feature of fixed block signalling is usually retained - the requirement for a full speed braking distance between trains. This ensures that, if the radio link is lost, the latest data retained on board the following train will cause it to stop before it reaches the preceding train. What distinguishes moving block from fixed block is that it makes the block locations and lengths consistent with train location and speed, i.e. making them movable rather than fixed.

Future Systems

The future signaling and train control system will very likely be a radio-based moving block system using a secure wireless data communication network to continuously track train location, speed and running direction. The subsystem would be required to utilize proven communication technologies to ensure data security and allow for interoperability with other systems. The system must also deliver robust performance under an adverse environment of high radio traffic and electromagnetic noises. It is also essential that the future system offers a low lifecycle cost and is able to overlay on any existing systems to facilitate system replacement.

  Core Domains and Technologies in Railway Signalling, Train Control and Communications
bulletTrain detection systems: Track circuits, Axle Counters, ...
bulletElectronic interlockings: SSI (Solid State Interlocking), ....
bullet Train protection systems: Country specific legacy systems: ATP, ATC, ..., International: ETCS (Europe), Cab signalling, Balise
bullet Formal Methods in Development and Verification of Interlockings and Distributed Railway Signalling and Control Systems
bullet Simulation of Railways and Interlockings
bulletCentralized network management and control: CTC (Centralized Traffic Control)
bulletIntegrated data/tele communication networks/solutions for railway signalling and control
bullet Communication-Based Train Control (CBTC)

Current systems: A complex railway signalling, train control and communication environment and interlocking based on standard data network

Far Vision 1: Convergence of rail traffic and road traffic managements: Intelligent Transportation Systems

Far Vision 2: Driverless ATC - make rail network safer, more reliable, and more productive than ever before without a driver

Central Office - ATS (Central ATC functions)

bulletMonitoring, display and control
bulletAutomatic planning & routing  (Planning creates a conflict-free timetable)
bulletVehicle regulation
bulletCoordination of traffic movement
bulletManagement of train schedules
bulletRecovery from timetable deviations
bulletUpdating reports and alarms
bulletEmergency stop requests

Wayside Subsystem - ATC  (safety-critical wayside functions)

bulletTrain detection
bulletTrain separation
bulletInterlocking control
bulletSpeed command transmission
bulletSpeed restrictions
bulletDirection control
bulletEmergency stop
bulletPlatform screen door interface

On-Board Subsystem - ATO/ATP (Driverless ATC’s automatic train operation)

bulletSpeed regulation 
bulletPlatform overshoot recovery
bulletOverspeed protection
bulletStation stopping
bulletDoor control
bulletPassenger information systems (including displays and audio information presented to passengers)
bulletRadio interface (e.g., emergency stopping of the train, putting a hold on the train, emergency door open, hold releases)
bulletRescue Operation
bulletVehicle Fault Detection and Response

Train control systems ensure safe & economic operations respecting the characteristics of the mode:

bulletPlanning creates a conflict-free timetable;
bulletControl ensures economic and dependable operations (the implementation of the timetable);
bulletControl and signalling interact to ensure safe train routing;
bulletSignalling and communication systems transmit information securely to drivers and signallers;
bulletTrack-to-train systems ensure safe movements (supervise).

The subsystem’s vital Vehicle Fault Detection and Response component detects on-board failures—in braking, propulsion, communication, doors, and more--that could occur and instantly responds with an appropriate countermeasure.

And, the entire subsystem features a fully redundant hardware and software architecture, so that if one unit fails, a backup unit can take over that particular control of the train. The result is increased reliability and availability every hour of every day.

  Key Questions About the Latest Signaling Technologies
bulletAre the relatively recent solutions sufficiently mature and proven?
bulletWill they satisfy tomorrow’s needs and regulations?
bulletHow will they affect short- and long-term investment, operating profits, value-added services and customer satisfaction?
bulletWhy and what collateral economic and social benefits are made possible by these technologies?
bulletWhat guarantees and strategies do solution suppliers offer to improve on-time, on-budget delivery of new signaling and resignaling projects without disrupting existing rail traffic?
bulletHow can we achieve a “low risk project”, or deploy a “future-proof technology”?
bulletHow can we apply the functional areas of network management, FCAPS, in complex railway network environments?

More recently, the words electronic, communication-based and software-intensive safety critical systems were joined in the rail industry’s vocabulary by open-standards, platform independent, interoperability, interchangeability, integrated diagnostics and logistical support, and many others previously reserved for high technology fields like the telecommunication, military and aerospace industries.

  Definitions from IEEE CBTC Standards

Automatic Train Control (ATC): The system for automatically controlling train movement, enforcing train safety, and directing train operations. ATC must include ATP, and may include ATO and/or ATS.

Automatic Train Operation (ATO): The subsystem within the automatic train control system that performs any or all of the functions of speed regulation, programmed stopping, door control, performance level regulation, or other functions otherwise assigned to the train operator.

Automatic Train Protection (ATP): The subsystem within the automatic train control system that maintains fail-safe protection against collisions, excessive speed, and other hazardous conditions through a combination of train detection, train separation, and interlocking.

Automatic Train Supervision (ATS): The subsystem within the automatic train control system that monitors trains, adjusts the performance of individual trains to maintain schedules, and provides data to adjust service to minimize inconveniences otherwise caused by irregularities. The ATS subsystem also typically includes manual and automatic routing functions.

Communications-Based Train Control (CBTC): A continuous automatic train control system utilizing high-resolution train location determination, independent of track circuits; continuous, high capacity, bidirectional train-to-wayside data communications; and trainborne and wayside processors capable of implementing vital functions.

bullet Signalling Resources


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This site was last updated 04/06/09