On Wed, 13 Aug 2003 13:00:07 +0000 (UTC), Mike Bristow
wrote:

In article ,
Ed Crowley wrote:
I wonder how complicated these signals are and why the same ones fail time
and time again after they have apparently been 'repaired'. For example, the
signals at Totteridge & Whetstone failed two days in a row recently.

"Signal failures" are usually "track circuit" failures.

The track circuit is the bit of the signalling system that detetects
the presence of a train; they get plumbed into the signal before
that section of track to turn the signal green [1]. They're designed
to fail-safe; ie if it's broke, the signal stays red even if it
could go green (this is considered better than the signal going
green when it should stay red, for obvious reasons).

I'd guess that repeated failures are due to a temporary fix failing
before the perminant fix can get done. Suppose that a track circuit
fails because a bit of wire has rotted. It might take a lot of
time and effort to replace that wire - it might be a couple of km
long! But you could patch the bit that's actually broke quite
quickly, so you do that and add "replace 2km of wire on the northern
line" to the List Of Things To Do Soon. The rest of the wire is
still in poor shape, so you may have more failures until you have
the time to replace it.

AFAIR, the signalling equipment on the Northern Line north of Camden
Town dates back to the 1940s. At least the kit in the Machine Rooms
does.


You patch, it fails, you patch, it fails....

Get the idea?


Resignalling will cure the problem.

Rob.
--
rob at robertwoolley dot co dot uk