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).
With joint less AFTC, electrical insulation is not necessary. Instead, detection of the section occupancy by a train is done by measuring the attenuation of the signal which is at a frequency (about 10kHz, usually) which does undergo significant attenuation in rails over the distances of interest and whose propagation characteristics are known. This also means that the entrance of a train into the track circuit section is not determined precisely based on its position -instead, safety factors are incorporated in the calculations to yield zones within which train occupancy can be determined in a guaranteed fashion.
As mentioned above, the signal is also coded in a pulse train allowing the receiver to distinguish between signals of different track circuit sections. In a variation of the jointless track-circuiting scheme, trackside units can be used to set up a resonant circuit and constrain the signal (usually between 1.5kHz and 3kHz) to a particular section of track.
The advantage of joint less AFTC is clear in that insulated joints are not required, reducing maintenance, allowing the use of long welded rail sections, and eliminating the problems of insulated joint failures. Jointless AFTC sections can be 1-1.5km in length. Jointless track circuits use audio frequency tuned circuits to create what amounts to a block joint. The media for the audio frequency signal is the rail and hence this system also detects for the broken rails.