 It uses 1700 Hz and 2300 Hz on one track and 2000 Hz and 2600 Hz on the other.To reduce the chance of stray currents causing a wrong side failure the modulation frequency is calculated by dividing the base frequency by 128.Different rates of modulation can be detected by equipment on the trains and used for ATC, so long as the transmitter end (Tx) is at the front of the train.