ABS

Automatic Block Signalling, abbreviated ABS, is a signalling regime where trains advance under the protection of fixed-length blocks of track, each block protected by a signal whose aspect is automatically governed by the occupancy of the blocks ahead. A green signal means at least two blocks ahead are clear; yellow means the block immediately ahead is clear but the next signal is at danger; red means stop. The dispatcher's role under pure ABS is limited to authorising movements onto the line — once a train is between two interlockings, the signals govern its progress without further intervention.

The strength of ABS is that it requires no human in the loop for routine block-to-block clearance. Track circuits — low-voltage electrical currents flowing through the rails — detect train wheels and automatically drop signals to danger as the train enters each block. The system is fail-safe by design: a broken rail, a power failure, or a damaged track circuit causes affected signals to fail to red, holding trains short of the trouble rather than letting them into it. ABS was the dominant North American signalling regime from the 1900s into the 1970s and still covers many secondary mainlines today.

The limitations of ABS are real. It cannot authorise opposing movements on single track — that requires either centralised CTC, or paper "manual block" authority issued by dispatchers. It cannot increase capacity beyond what the fixed block geometry allows. And it cannot route trains over junctions without an interlocking superimposed at every diverging point. Modern signalling on capacity-constrained mainlines therefore tends to combine ABS-style automatic block protection on plain track with CTC at every interlocking — the best-of-both arrangement that dominates current North American practice.

For railfans, ABS territory is identifiable by the regular spacing of intermediate signals between interlockings, each on its own bracket or mast and showing aspects that change independently of any dispatcher action. Walking a stretch of ABS line on foot reveals dozens of signals in a chain, each governing its piece of the next mile.