Principles and Operation
The basic principle behind the track circuit lies in the connection of the two rails by the wheels and axle of locomotives and rolling stock to short out an electrical circuit. This circuit is monitored by electrical equipment to detect the absence of the trains. Since this is a safety appliance, fail-safe operation is crucial; therefore the circuit is designed to indicate the presence of a train when failures occur. On the other hand, false occupancy readings are disruptive to railroad operations and are to be minimized.
Track circuits allow railway signalling systems to operate semi-automatically, by displaying signals for trains to slow down or stop in the presence of occupied track ahead of them. They help prevent dispatchers and operators from causing accidents, both by informing them of track occupancy and by preventing signals from displaying unsafe indications.
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