railway signalling Railway signalling (), also called railroad signaling (), is a system used to control the movement of railway traffic. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormo ...

, a treadle is a mechanical or electrical device that detects that a train wheel has passed a particular location. They are used where a

track circuit A track circuit is an electrical device used to prove the absence of a train on rail tracks to signallers and control relevant signals. An alternative to track circuits are axle counters. Principles and operation The basic principle behind t ...

requires reinforcing with additional information about a train's location, such as around an automatic

level crossing A level crossing is an intersection where a railway line crosses a road, path, or (in rare situations) airport runway, at the same level, as opposed to the railway line crossing over or under using an overpass or tunnel. The term a ...

, or in an annunciator circuit, which sounds a warning that 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

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.

Types

Mechanical

In situations where track circuits are unreliable due to rusty rails, for example adjacent to

buffer stop A buffer stop, bumper, bumping post, bumper block or stopblock (US), is a device to prevent railway vehicles from going past the end of a physical section of track. The design of the buffer stop is dependent, in part, on the kind of couplings ...

s and

catch points Catch points and trap points are types of turnout which act as railway safety devices. Both work by guiding railway carriages and trucks from a dangerous route onto a separate, safer track. Catch points are used to derail vehicles which are ou ...

, a long treadle bar is used. When this is depressed, the signaller gains indication (if they have 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 An axle counter is a system used in railway signalling to detect the clear or occupied status of a section of track between two points. The system generally consists of a wheel sensor (one for each end of the section) and an evaluation unit for c ...

circuits that can replace track circuits completely.

Variations

Variations on a treadle that can be one 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 In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

to 'stop' can be replaced by a short

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

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especially to put starting signals to danger. 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 The Hawes Junction rail crashHawes Junction station is now called Garsdale occurred at 5.49 am on 24 December 1910, just north of the Lunds viaduct between Hawes Junction (now known as Garsdale station) and Aisgill on the Midland Railway's ...

Level crossing

Treadles are commonly used to operate fully automatic

s 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.

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 signal to return to danger during a busy period that led to the worst accident on that railway, the Clayton Tunnel rail crash. Because the signal had failed to return to stop with another train approaching close behind, the signalman was forced to wave a red flag at the oncoming train without knowing if the train would see this. After wrongly assuming the train had then left the tunnel, the signalman allowed a further train to enter the tunnel, where it collided with the first which had started reversing. The biggest flaw in the Whitworth automatic signal is probably that it 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.Southern Signals
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