A Phase Shift Overlay, or PSO, track circuit is an advanced AC track circuit that detects trains by monitoring changes in the phase of an alternating current signal transmitted through the rails.
Unlike traditional AC track circuits that mainly depend on signal amplitude, PSO systems compare the phase of the received signal to a reference signal. A change in phase can indicate that the track circuit condition has changed.
PSO systems continuously transmit a low-power AC signal at a specific frequency through a defined track block. The receiver at the other end measures the phase of the signal and determines whether the block is clear or occupied.
A PSO track block may use tuned circuits or impedance matching units at the boundaries instead of relying only on insulated rail joints. These devices help define the electrical limits of the PSO signal while allowing other currents or frequencies to pass where required.
When a train enters the block, the wheels and axles change the impedance of the track circuit. This impedance change causes a phase shift in the received AC signal, and the receiver detects that shift as an occupied track condition.
PSO circuits can also support broken rail detection. A break in either rail can significantly change the impedance of the track circuit, creating a detectable phase shift and indicating a potentially unsafe condition.
Phase shift overlay circuits can often be used over longer distances and may be overlaid on existing signaling systems, making them useful in complex railroad environments.