As railway systems evolved, especially with electric traction, the limitations of DC track circuits became more noticeable. AC propulsion return currents can interfere with DC track circuits, so alternating current track circuits were developed to help reduce that interference.
Like DC track circuits, AC track circuits use the rails as conductors to detect the presence of a train within a defined track block. Instead of direct current, they use an alternating current signal, usually at a frequency selected to avoid interference from traction power.
In an AC double rail track circuit, both rails are insulated from adjacent track sections using insulated rail joints. An AC power source is connected to the rails at one end of the block, and an AC-sensitive relay or receiver is connected at the other end.
When the track section is clear, AC current flows through the rails to the receiver, energizing it and indicating that the track is unoccupied.
When a train enters the block, the wheels and axles create a low-impedance shunt across the rails. This shunt diverts current away from the receiver, causing it to de-energize and indicate that the track is occupied.
A key component in AC double rail track circuits, especially in electrified territory, is the impedance bond. Impedance bonds allow traction return current to continue flowing through the rails while helping prevent the track circuit signal from being shorted or interfered with.