On 18 Sep, 11:10, Sim wrote:
On 18 Sep, 10:47, MIG wrote:





On 18 Sep, 09:44, "John Salmon" wrote:

"Sim" wrote Now let's be nice to each other! I did not know the details Charles

kindly provided, but it does make sense. Interestingly, not only does
fourth rail (however wired) exist as far north as Harrow, but the last
time I looked there was quite a lot left further on, although some of
it was lying rather dismally in the four foot rather than perched on
insulators. It was never formally removed, in other words, although
doubtless disconnected.

Previous discussions have suggested that it is still there because it
is still connected. *I think the running rails must be wired to the
fourth rail and then the fourth rail wired to earth in the way that
the running rails are in other third rail systems. *(Not technical,
but I assume that it saved connecting the running rails for one sort
of train and duplicating the connections for another sort of train.)

Question: was this the system at Euston and Broad Street etc, ie
positive to earth, rather than a bit positive to a bit negative?

Presumably for an LU train, the difference between the rails is all
that matters, whereas for a three-rail train it's the difference
between positive and earth that matters, so a NR train on conventional
LU track would only be at 420 V or something?

I really doubt that the old fourth rail is in circuit with anything.
Some is missing, and other sections are lying in the four foot. Not
much continuity there, I would have thought!

It is still wired to the running rails, if you look from a train at
the other track, you can see the connections. The running rails were
never the continuous electrical return and were not bonded together.
The fact that the rail lies in the four foot doesn't matter as it is
earthed, missing sections will have been bypassed with cabling and
very little is actually missing if you look for it. If nothing else,
it has to be connected to something, due to possible induction from
the 25kV AC running along side.