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Absolute Signal

A block or interlocking signal without a number plate, or designated by an A marker.

Automatic Block Signal System (ABS)

A series of consecutive blocks governed by block signals, cab signals, or both. The signals are activated by a train or by certain conditions that affect the block use.


A length of track:
• between consecutive block signals.
• between a block signal and the end of block system limits.
• in ATC limits the use of which is governed by cab signals and/or block signals.

Block Signal

A fixed signal at the entrance of a block that governs trains entering and using that block.

Block System

A block or series of consecutive blocks within ABS, ACS, CTC, or interlocking limits.

Centralized Traffic Control (CTC)

A block system that uses block signal indications to authorize train movements.

Control Point

The location of absolute signals controlled by a control operator.

Current of Traffic

The movement of trains in one direction on a main track, as specified by the rules.

Distant Signal

A fixed signal outside a block system that governs the approach to a block signal, interlocking signal, or switch point indicator. A distant signal does not indicate conditions that affect track use between the distant signal and block or interlocking signals or between the distant signal and switch point indicator. A distant signal is identified by a D.

Double Track

Two main tracks where the current of traffic on one track is in a specified direction and in the opposite direction on the other.

General Track Bulletin

A notice containing track bulletin restrictions and other conditions affecting train movement.


Signal appliances that are interconnected so that each of their movements follows the other in a proper sequence. Interlockings may be operated manually or automatically.

Main Track

A track extending through yards and between stations that must not be occupied without authority or protection.

Multiple Main Tracks

Two or more main tracks that are used according to the timetable.

Proceed Indication

Any block signal indication that allows a train to proceed without stopping.

Track Warrant Control (TWC)

A method to authorize train movements or protect men or machines on a main track within specified limits in a territory designated by the timetable.

Trackside Warning Detector

A device that indicates conditions such as overheated journals, dragging equipment, excess dimensions, shifted loads, high water, or slides.


A system of tracks, other than main tracks and sidings, used for making up trains, storing cars, and other purposes.

Yard Limits

A portion of main track designated by yard limit signs and timetable special instructions or a track bulletin.

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