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Railway Signalling Terms

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Absolute Block
A system of controlling rail traffic, where (under normal operations) only one train is allowed in the Block Section at a time. Proof of a section clear normally involves the observation of the train tail lamp by the Signaller.

A device that can measure acceleration generated by the movement of an object along an axis.

Access Point
A device that allows wireless devices to connect to a wired network using Wi-Fi.

Automatic Route Setting (ARS)
A system for setting Routes without the action of the Signaller, based upon a stored timetable, train running information, defined priority, selection criteria and operating algorithms.

Automatic Signal
A Signal controlled by the passage of trains. It does not require any action by the Signaller or ARS. Automatic Signals are usually Passable.

Automatic Track Warning System (ATWS)
A system that gives trackside staff audible and/or visible warning of the approach of trains independently of the Signalling System.

Automatic Train Control (ATC)
Used to describe on-board automation that contributes to or replaces the driver’s judgement as to how to control the train. (ATC=ATO+ATP)

Automatic Train Operation (ATO)
A high reliability system that automatically operates the train’s driving controls in accordance with information usually received from the trackside signalling equipment or traffic control system.

Automatic Train Protection (ATP)
A safety system that enforces either compliance with or observation of speed restrictions and/or Signal Aspects by trains.

Automatic Train Regulation (ATR)
A subsystem to ensure that the train service returns to timetabled operation or to regular, fixed headways, following disruption. ATR subsystems adjust the performance of individual trains to maintain schedules. ATR is normally a subsystem of automatic train supervision (ATS).

Automatic Train Supervision (ATS)
A safety within an automatic train control system which monitors the system status and provides the appropriate controls to direct the operation of trains in order to maintain intended traffic patterns and minimize the effect of train delays on the operating schedule.

Automatic Warning System (AWS)
A system that provides audible and visual warnings to the driver on the Approach To Signals, certain Level Crossings and Emergency, Temporary and certain Permanent, Speed Restrictions. A track Inductor based system linked to the aspects of fixed lineside Signals. The track mounted inductors are supplied as standard or extra strength.

Axle Counter
An axle counter is a device on a railway that detects the passing of a train between two points on a track. Track mounted equipment counts the number of axles entering and leaving a Track Section at each extremity. A calculation is performed to determine whether the track section is Occupied or Clear.

A track mounted spot transmission unit that uses transponder technology. Its function is to transmit/receive messages to/from the train passing overhead.

A location where a Train Description may be displayed by the Train Describer and which is normally associated with a Signal.

Braking Curve
A graphical representation of the Braking Distance of a train in relation to the Gradient, the braking characteristics and speed of the train.

Braking Distance (Emergency)
The distance in which a train is capable of stopping in an emergency. Dependent upon train speed, train type, braking characteristics, train weight and/or gradient.

Braking Distance (Service)
The distance in which a train is capable of stopping, from a given speed, at such a deceleration for a passenger train that the passengers do not suffer discomfort or alarm, or at an equivalent deceleration in the case of non-passenger trains.

Cab Secure Radio (CSR)
A secure radio communication system between driver and Signaller.

Cab Signal
A display in the driving cab of a train, showing Permissible Speed or extent of Movement Authority, instead of or supplementing lineside Signals.

The authority given by a Signaller to a driver to pass a Signal at Danger.

Communications-Based Train Control (CBTC)
A railway signaling system that makes use of the telecommunications between the train and track equipment for the traffic management and infrastructure control. By means of the CBTC systems, the exact position of a train is known more accurately than with the traditional signaling systems.

Computer Based Interlocking (CBI)
A generic term for a second generation processor based system for controlling the Interlocking between Points and Signals, as well as communication with lineside Signalling Functions.

Degraded Mode Conditions
The state of the part of the railway system when it continues to operate in a restricted manner due to the failure of one or more components.

An employee who supervises the train movements of a line or a certain area. A dispatcher may also perform the duty of a Train Control Operator.

Driverless Train Operation (DTO)
A signal train operation where starting and stopping are automated but a train attendant operates the doors and drives the train in case of emergencies, or GoA3.

Dwell Time 
The total elapsed time from the time that a train stops in a station until the time it resumes moving.

Emergency Release
A device, usually sealed, to permit the operation of a Signalling Function in case of emergency or failure.

European Rail Traffic Management System (ERTMS)
A system for managing rail traffic, enabling it to operate on compatible Signalling Systems across European borders.

European Train Control System (ETCS)
The train control part of ERTMS.

  • Level -1; An intermittent ATP system following the ETCS standard that uses controlled Eurobalises for transmission of control data.
  • Level -2; An continuous ATP system following the ETCS standard that that combines radio-based train control with a fixed block system.
  • Level -3; An continuous ATP system following the ETCS standard that that combines radio-based train control with radio-based train separation based on moving block or virtual block.

A standardised balise for use in the European Train Control System.

A design philosophy which results in expected failures maintaining or placing the equipment in a safe state.

Fixed Block
The section of track between two fixed points.

Flank Protection
Protection from overrunning movements approaching on converging tracks, usually by additional Point Interlocking or Train Detection.

Grades of Automation (GoA)
There are various degrees of automation; these are defined according to which basic functions of train operation are the responsibility of staff, and which are the responsibility of the system itself.

Any real or potential condition that can cause injury, death, or damage or loss of equipment or property.

An indication given to the driver of a train during Shunting Movements or in other exceptional circumstances to control the movement of the train.

The shortest distance or time interval between two following trains, so that the second train can run at its normal operating speed without being restricted by the Signal Aspects.

An electrical, electronic or mechanical means of making the operation of one piece of apparatus dependent upon certain predetermined conditions being fulfilled by other apparatus. The logic by which routes that conflict are prevented from being set at the same time.

The normal state of a Block Section when no permission has been given for a train to enter it.

Line Blocked (LB)
The normal state of a Block Section when no permission has been given for a train to enter it.

Line Capacity
For a given section of line, the practical maximum number of trains per hour permitted by the Signalling System.

Line Clear (LC)
The state of the Block Section after a train has been accepted but before it has entered the block section.

Line Clear
Release The Signaller can only pull the lever for the Section Signal if Line Clear is obtained from the box ahead. The lever is released either for One Pull or One Train.

Line Speed
Obsolete term for Permissible Speed.

Moving Block
A signalling block system where the blocks are defined in real time by computers as safe zones around each train.

A Signal is designated Non-Passable because it protects an area of conflict or other infrastructure such that a significant hazard would arise in the event of it being passed at danger without authority. Such Signals cannot be passed at danger without specific authority from the Signaller, in accordance with the Rule Book. Non-passable Signals are usually Controlled Signals.

Related A description applied to those parts of the Signalling System whose failure or non-availability does not directly endanger rail traffic or reduce the integrity of the Signalling System.

Signalling equipment and circuits are considered non vital where failure to function correctly would not cause an unsafe outcome of the signalling system. Non-vital equipment and circuits do not affect the safe operation of the signalling system.

A device that measures the distance travelled based on wheel revolutions, radar and/or accelerometers.

Operations Control Center (OCC)
A location or locations designed, equipped, and staffed for the purposes of monitoring and controlling rail transit service activities from a central location or locations.

Overlap (OL)
The distance beyond a Stop Signal that must be clear, and where necessary Locked, before the Stop Signal preceding the Stop Signal in question can display a Proceed Aspect.

Point Machine (or Switch Machine)
A machine that is used to operate points, movable frogs or derailers.

Radio Block Centre
A control centre to supervise and control train movements in a territory with radio-based train control.

Safety Integrity Level (SIL)
Defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction.

Semi-automatic Train Operation (STO)
A signal train operation where stopping is automated but a driver in the cab starts the train, operates the doors, drives the train if needed and handles emergencies. GoA2

A track structure of movable running rails (points) with necessary fastening to provide a means for routing trains from one track to another.

An instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine.

A document that contains the schedules of all trains of a line.

Track Circuit (TC)
An electrical device using the rails in an electric circuit, which detects the absence of trains on a defined section of line.

Track Section
A portion of railway track having fixed boundaries and for which the Train Detection System provides information on its state of occupancy to the Signalling System.

Train Detection System
Equipment and systems forming part of, or providing input to, the Signalling Systems to detect, either:

  • the presence or absence of vehicles within the limits of a track section, or
  • that a train has reached, is passing or has passed a specific position.

Where required, a train detection system may additionally detect the direction in which a train is travelling.

A transponder is a wireless communications, monitoring, or control device that picks up and automatically responds to an incoming signal. The term is a contraction of the words transmitter and responder. Transponders can be either passive or active.

Turnout Speed
The speed permitted through the Facing Points when Set for the Diverging Route.

Unattended Train Operation (UTO)
A signal train operation where starting and stopping, operation of doors and handling of emergencies are fully automated without any on-train staff, or GoA4.

Vehicle On-Board Controller (VOBC)
It establishes the position of the train on the guideway by detecting transponders located in the track bed, and uses the transponder data to extract information from the database. Database on the Vehicle On-board Controller contains all relevant guideway information, including station stops, gradients, civil speed limits, switch locations, axle counter blocks locations and trackside signal locations.

Equipment whose correct operation is essential to the integrity of the Signalling System. Most vital equipment is designed to Fail-Safe principles – a Wrong Side Failure of vital equipment could directly endanger rail traffic.

Vital Function
A function in a safety critical system that is required to be implemented in a fail-safe manner.

Sources: rssb.co.uk; metroautomation.org; nsw.gov.au; joernpachl.de