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 re-inforcing 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. 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 pin-point detection.

Types

Mechanical

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.

Electro-Mechanical

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.

Electronic

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

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.

Reverser

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.

Level crossing

Treadles are commonly used to operate fully automatic level crossings since they give far more reliable and accurate detection of a train than track circuits alone, which is important when there is only just over 30 seconds between the train "striking in" (passing the treadle which starts the crossing sequence) and passing the crossing.

Greasers

Greasers use a small treadle to apply a small quantity of grease to the inside edge of the rail to reduce friction and noise between the flange of the wheel and the rail. [http://www.semgonline.com/infrastr/track_05.html]

Accident

One early signal was the "Automatic" signal invented by CF Whitworth. Far from being "automatic" in operation, this was merely a signal that was operated by the signalman but returned to 'danger' once the train had passed, by means of a treadle. There was one of these at each end of Clayton Tunnel, just north of Brighton, and it was the failure of the signalman to see that the signal had not returned to danger that led to the worst ever accident on that railway. Because the signal had failed to return to stop, a second train entered the tunnel and collided with the first which had stalled.

The biggest flaw in the Whitworth automatic signal is that it probably had no redundancy, and a single stone might jam it. On the other hand, without a treadle, the signalman is more likely to get distracted and forget to put the signal to stop. [ [http://www.semgonline.com/proto/signals.html Southern Signals] ]

References