Axle counters are used at the ‘farpoints’ of the approaches (typically 1.200 metres from the crossing), ‘nearpoints’ (about 600 metres away), and at the ‘island circuit’ of the crossing (about 60 metres on each side of the road). Additional wheel detection points may be used to monitor switches or multiple track systems where required.
Two wheel sensors are placed 2 metres apart at each detection point to compensate for variations in the depth and position of the wheel flange. This creates space and time diversity which mitigates wheel hunting, electrical noise, and vibrations to ensure both wheels of the axle are counted.
As wheels are counted by the two separated sensors, the speed is measured extremely accurately. Train position is calculated based on the speed-time integration, allowing the calculation of the progression into the crossing zone. This provides a constant warning time of e.g. 25 seconds for any train speed.