 To accommodate this, AC track circuits use alternating current signals instead of direct current (DC) but typically, the AC frequency is in the range of audio frequencies, from 91 Hz up to 10 kHz.The relays are arranged to detect the selected frequency and to ignore DC and AC traction frequency signals.Again, failsafe principles dictate that the relay interprets the presence of the signal as unoccupied track, whereas a lack of a signal indicates the presence of a train.