On 21/09/10 18:15, The Gardener wrote:

On the Southern, both running rails are used for traction return.
Track circuits are AC as a result; historically 50 Hz but modern track
circuits (known as TI for Traction Immune) use higher frequencies (I
believe in the range 1.1-1.3 kHz) to avoid the risk of harmonics in
the return current giving a false clear indication.

I thought modern track circuits transmit a digital code so as to
completely eliminate the risk of traction systems generating the
appropriate frequency?

-roy