Treadle (railway)
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On a railway, a treadle is a device that detects the passing of a train, a bit like a track circuit, and might be used to put a railway signal to stop. The earliest treadles were mechanical devices which looked a bit like a pedal, hence the name, although later electrical and electronic versions lack this physical feature.
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[edit] Rail vehicle detection
An important difference between the treadles and track circuits is that while a track circuit detects a train over a distance as long as several kilometres, a treadle provides pin-point detection.
Treadles are useful in situations where track circuits are unreliable due to rusty rails, for example adjacent to buffer stops and catchpoints, or where trains are short with low axle loads.
[edit] Types
Treadles can be mechanical, electro-mechanical, or a non-contanct electronic type. The mechanical type has the pedal that gives the treadle its name.
[edit] 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.
[edit] 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.
[edit] Axle counters
Treadles are used in axle counters.
[edit] 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, 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. They are also much cheaper and simpler than providing multiple track circuits, particularly on lines with very long signal sections but many crossings. On the St Albans Branch Line there is an AHB level crossing outside Watford North operated by a treadle and a manual plunger for trains towards St Albans, and by a treadle for trains towards Watford Junction.[citation needed]
[edit] 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. [1]
[edit] 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 Policeman 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 Policeman 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 policeman (or signalman) is more likely to get distracted and forget to put the signal to stop. [1]
