An AC double rail track circuit is a train detection system that uses alternating current and has both rails electrically isolated from adjacent track sections by insulated rail joints. This creates a defined track block with clear electrical boundaries on both running rails.
An AC power source is connected across the two insulated rails at one end of the block. An AC-sensitive relay or electronic receiver is connected across the rails at the opposite end.
When the track section is clear, the AC signal flows from the source through the rails to the receiver. The receiver energizes and indicates that the track section is unoccupied.
When a train enters the block, the metal wheels and axles create a low-impedance shunt across the two rails. This shunt provides an easier path for current and reduces the current reaching the receiver.
When the receiver de-energizes, the track circuit indicates that the track section is occupied. This is the basic train detection principle used by AC double rail track circuits.
The term double rail means that both rails are part of the signaling circuit and both rails are insulated at the block boundaries.