Track Circuit

an isolated segment of a railroad line that is part of the railroad signaling system; track lines serve as current conductors in this circuit. These isolated railroad segments are called blocks. Blocks are track sensors that are tripped by the action of the wheels of the rolling stock, thus establishing a contact between the rolling stock and the control devices—the railroad switches and signals. When the track circuit is free of traffic, the current of the track battery passes through the track relay, whose contacts close the circuit for the green light signal. When the wheels of the rolling stock pass onto a track circuit, the track relay is shunted and the armature of the track relay drops, which results in a red or a red-yellow light signal.

In order to check that the track circuit is free of traffic, a signal current is sent through the circuit—direct currents and alternating currents are determined by the type of track circuit. Circuits that are normally closed and those that are normally open are determined by their operating principles. An operable track circuit is considered to be in a normal state when it is free of rolling stock.

When the track circuit is of a normally closed type, the current is sent continuously; thus, in addition to its basic functions, the circuit also checks the working order of the track devices, including the track line. When the track circuit is normally open, the track relay is usually not activated and does not check the working order of the circuit. Except in marshaling yards, only track circuits that are normally closed are used on railroad lines in the USSR.