Clive D. W. Feather wrote:

Extremely difficult.

The basic design of a track circuit is very simple:

Supply Relay
| | | |
| | | |
=:=+==========+= =+===========+= =+==========+= =+=======+==:==
| \---/ \---/ \---/ |
/--/ \-\
| /---\ /---\ /---\ |
=:=+==========+= =+===========+= =+==========+= =+=======+==:==

=== running rail
=:= insulated gap
= = ordinary gap between rails
=+= wire bonded to rail
|

The supply will be DC in some areas, but AC at a specific frequency
on LU. The relay will be tuned to the same frequency (so a feed from
an adjacent track circuit won't trigger it).
The wheels and axles of the train short the rails, causing the
relay to de-energise. Any fault in the wiring causes the relay to
de-energise. But to get a false clear on the relay you've got to
feed significant amounts of 83 1/3 Hz (or whatever) current into
the circuit to the right of the location of the train.

Ah, I see. I'd never actually seen a proper diagram of a track circuit
before; this makes things nice and clear.


More that "double fault" is a general principle. In-the-field
circuits are often double cut (that is, both supply and return are
switched by the controlling relay) so that a false feed or false
earth doesn't trigger it.

OK.


Are you talking about a specific event, or just a general enquiry?


I'm making a general inquiry. Most of the stuff I've read from District
Line drivers states that the slightest wet down there causes massive
problems with the signalling. I was just curious as to whether or not
those problems could cause a wrong-side failure.