Automatic block signaling

The automatic operation comes from the system's ability to detect whether blocks are occupied or otherwise obstructed, and to convey that information to approaching trains.

[3][4]: 6–9 The first use of automatic block signalling in the United Kingdom was installed on the Liverpool Overhead Railway on its opening in 1893.

Instead of track circuits, the system used a setup of trackside mechanical, and later, electrical instruments (both functionally similar to treadles) that made contact with passing trains in order to trigger motor-operated mechanical signals.

[5]: 103  The first use of track circuit operated automatic block signalling in Britain was installed in 1902 by the LSWR's West of England line between Andover junction and Grateley which operated pneumatically powered mechanical signals.

[4]: 8 However, as time went on, many railroads came to see automatic block signaling as cost effective, since it reduced the need for employees to manually operate each signal, reduced the repair costs and damage claims resulting from collisions, made possible a more efficient flow of trains, reduced the number of hours trains and crews sat idle, and decreased overall transit times from point to point.

In the UK, automatic signals are used where there are no ground frame, flat junctions, railroad switch facing and trailing, manually controlled level crossing, neutral sections, or other interlocking functions.

Searchlight type automatic block signal at Milepost 122.2 on the Reading Blue Mountain and Northern Railroad Lehigh Line (former Lehigh Valley Railroad ) in Jim Thorpe, Pennsylvania
A caution signal at ME 130 at Beaconsfield station in England, where the horizontal black bar at the top of the ID plate indicates an automatic signal [ 6 ]
A pair of ABS position light signals each governing one direction of travel on Norfolk Southern Railway 's Enola Branch (former Pennsylvania Railroad )