On Sep 22, 12:41*pm, Roy Badami wrote:
On 21/09/10 18:15, The Gardener wrote:

On the Southern, both running rails are used for traction return.
Trackcircuits are AC as a result; historically 50 Hz but moderntrack
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 moderntrackcircuits transmit a digital code so as to
completely eliminate the risk of traction systems generating the
appropriate frequency?

* * *-roy

You're thinking of the HVI (High Voltage Impulse) type. These are used
where there is a risk of low wheel-rail adhesion causing the train to
"disappear" from the system. TI types are generally more common.

HTH.