Two sets of two electronic loops are set in the track before signals where a SPAD would be most likely to put a train into risk of being involved in a collision. The loops are energised when the signal is red and emit electromagnetic fields. When an antenna on the train detects the field emitted by the first loop of a pair it starts an electronic timer. The field emitted by the second loop of the pair stops the timer. The first pair of loops encountered are spaced so that the timer on the train can calculate whether the safe speed for approaching the signal is being exceeded. If it is, the brakes are automatically applied. The second pair of loops are situated closer to the signal and are placed very close together such that the safe speed is effectively zero. TPWS will be used in conjunction with the existing AWS.
