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Train detection is achieved by means of track switches placed at intervals along the line. As the train's wheels pass over the switch, the wheel flange depresses an arm which operates a micro-switch. Generally switches are placed a train length apart, the maximum train length being limited by the length of the turntable in the station. As a train passes over a switch the relay for that section of line is de-energised, turning the signal red. When the train is clear of the section it passes over another switch which re-energises the relay allowing the signal to clear again. Most switches will actually carry out both functions i.e. de-energising the relay for this section and re-energising the relay for the previous section.