AC double rail track circuits are used in railroad signaling because they are better suited for certain railway environments, especially electrified territory where traction return current flows through the rails.
In electrified railways, traction power can create return currents that travel through the rails or dedicated return paths. These currents can interfere with DC track circuits and may cause unreliable operation, false indications, or detection problems.
AC double rail track circuits use an alternating current signal that can be selected and tuned to operate reliably even where traction return current is present. This makes them more compatible with AC traction territory than basic DC track circuits.
Impedance bonds are an important part of many AC double rail track circuits. They allow traction return current to continue flowing through the rails while helping protect the track circuit signal from being shorted or disrupted.
This helps reduce false indications and improves the reliability of train detection. A properly designed AC double rail track circuit can provide accurate track occupancy information even in areas with heavy traction power currents.
AC double rail track circuits may also offer improved performance in some non-electrified areas where environmental conditions, leakage, or interference make DC track circuits less reliable.