On Wed, 22 Sep 2010 12:41:37 +0100, Roy Badami
wrote:

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?

Some possibly do but it can take many years for innovations to spread
when you consider that semaphore signalling is still in use long after
the introduction of colour-light signalling.