What kinds of track circuits are used by IR?
The most common form of track circuit used is the detection of a train by the closing of an electrical circuit between the two rails because of the conducting nature of the rolling stock. This circuit may use DC in the simplest form, or may use AC. Most zones now have many sections that use AFTC, or Audio-Frequency Track Circuit, that sends modulated electrical signals at a particular frequency (a few kHz, often 831.33Hz) through the rails rather than relying on a simple circuit connectivity; this is more reliable and allows the track circuit length to be increased a lot. The pioneers in adopting AFTC over simple DC track-circuiting were WR, SR, and CR (Dombivli, Pune-Lonavala, Chennai-Tambaram, Anand-Vatva, etc.). CR has also experimented with a variant known as the High-Frequency Track Circuit (HFTC). Track circuits may also be ‘coded’, which means that the transmitted signal used in the track circuit is not a steady one (either DC or AC) but forms a pulse train of a particular patte
