When there is no train running in the blocked area, the current will flow from the power source to the relay through the track, and it will activate the magnetic contact to turn on the green light circuit (the signal immediately indicates safe passage). When a train enters the blocked area, the current diverts through the train axle, and does not flow through the relay. The relay loses its magnetism due to the loss of current, and the contact is connected to the circuit of the red light (the signal immediately shows the danger and prohibition). If the track is broken, the track circuit will be blocked and the relay will lose its magnetism. When the train leaves the entire section, the relay will be re-energized, the green light will turn on again, and other trains can enter.
When a certain section of the line with the track circuit is idle; the relay on the track circuit has enough current to pass through, suck up the magnetized armature, and close the front contact, thereby turning on the green light circuit of the color light signal, and displaying the green light, Indicates that the line ahead is free, allowing rolling stock to occupy it. When the rolling stock enters the line section, due to the small resistance of the wheelset, the track circuit is short-circuited, the suction of the relay is weakened, the armature is released, and the armature is placed on the rear contact, the red light circuit of the signal is turned on, and the prohibition signal is displayed. . This working performance of the track circuit can prevent train rear-end collisions and conflict accidents and ensure driving safety.
Another important function of the track circuit is to detect the fracture of the rail. When the rails that act as conductors are safe and sound, the rail currents flow unimpeded and the relays work normally. Once the front rail is broken or there is an obstruction, the rail current is cut off, which will cause the relay to release the armature due to insufficient power supply and connect to the red signal circuit. At this time, although the line is idle, the signal still shows the red light, thus preventing the train from overturning the accident.
