Treadle (railway)

In railway signalling, a treadle is a mechanical or electrical device that detects that a train axle has passed a particular location. They are used where a track circuit requires reinforcing with additional information about a train's location, such as around an automatic level crossing, or in an annunciator circuit, that sounds a warning a train has passed an exact point. They also serve as a critical backup in the case of track circuit failure. The important difference between a treadle and a track circuit is that while a track circuit detects a train over a distance as long as several kilometres, a treadle provides detection at a single fixed location.

In situations where track circuits are unreliable due to rusty rails, for example adjacent to buffer stops and catchpoints, a long treadle bar is used. When this is depressed, the signalman gains indication (if he has not already done so) of a train in a section.

An electro-mechanical treadle retains a small arm that lies across the flangeway of a rail. When it is depressed, an electrical circuit controller within the unit changes its output. It remains depressed for a period of several seconds, so that a train with many axles does not unduly damage the unit.

An electronic treadle uses the disruption of an electromagnetic field to detect an axle, rather than a depression bar. Hence, it can count individual axles. Electronic count heads are used in axle counter circuits that can replace track circuits completely.

Variations on a treadle that can be carriage long include facing point lock bars, clearance bars, and train bars, depending how they are located on a track layout.

A mechanical treadle that puts a signal to 'stop' can be replaced by a short track circuit and a reverser. A reverser is an electrically engaged latch that allows the signal to be reversed, i.e. placed to green. When the track circuit past the signal is occupied, power to the latch is removed, and the signal reverts to 'stop', red.

Reversers have been used in New South Wales especially to put starting signals to stop. When the signal was put back to stop, the system automatically sent the train on line bell signal (two bells) to the station in advance. This equipment would have helped prevent the Hawes Junction rail crash.

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Wikipedia