Axle counters

An axle counter is a device on a railway that detects the passing of a train between two points on a track. A counting head (or 'detection point') is installed at each end of the section, and as each train axle passes the counting head at the start of the section, a counter increments. A detection point comprises two independent sensors, therefore the device can detect the direction and speed of a train by the order and time in which the sensors are passed. As the train passes a similar counting head at the end of the section, the counter compares count at the end of the section with that recorded at the beginning. If the two counts are the same, the section is presumed to be clear for a second train.

This is carried out by safety critical computers called 'evaluators' which are centrally located, with the detection points located at the required sites in the field. The detection points are either connected to the evaluator via dedicated copper cable or via a telecommunications transmission system. This allows the detection points to be located significant distances from the evaluator. This is useful when using centralised interlocking equipment but less so when signalling equipment is distributed at the lineside in equipment cabinets.

The most common use for axle counters is within railway signalling for track vacancy detection. This is a form of block signalling, which does not permit two trains to be within the same block at the same time, this decreases the chance of collision due to railways being divided into blocks which ensures there is always enough space between trains to allow one to stop before it hits the one in front.

Axle counters are also used for the switching on and switching off of railway crossings. Closing the crossing to pedestrian and motor vehicles when the presence of a train is detected, and allowing them to open back up when the successful traverse of the train has been recorded.

Axle counters are used in rail yards to detect train cars as they are sorted. Axle counters are placed on the track before each switch and on each track that exits the switch. Rail yard management software uses occupancy data from the axle counters to lock in switches and prevent cars from being routed to tracks that are occupied by other cars.

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Wikipedia