Type C track circuits are an advanced form of AC track circuit technology used for reliable train detection and broken rail detection in difficult railroad environments.
They are especially useful in electrified territory where high levels of AC traction return current can interfere with simpler track circuit designs.
A key feature of Type C track circuits is that they often use two different AC frequencies transmitted through the rails at the same time.
One frequency may be used primarily for train detection, while another frequency may be used for broken rail detection. The receiver uses filters and detection circuits to analyze these frequencies separately.
When a train enters the block, its wheels and axles shunt the track circuit and change the received signal characteristics used for train detection.
A broken rail can affect the second frequency by changing the impedance, signal level, or phase of the circuit. This allows the system to detect a possible broken rail even when no train is present.
Type C track circuits are known for strong immunity to AC traction interference because they use distinct frequencies and filtering techniques. Although they are more complex than simpler AC or DC circuits, they provide high reliability in demanding signaling environments.