A track circuit is a device used to detect the presence of a train on a railway track. It consists of two electrical circuits connected to each other, one of which is connected to the track and the other to the train. The two circuits are connected by a LAP, which is a metal loop that is placed around the track. The term 'track circuit' can also refer to a single circuit, such as the ONELAP, which is a single-circuit track circuit that uses a single LAP to detect the presence of a train.