"MIG" wrote in message
...
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?

Or nothing - there should be "no circuit", but I suspect could cause some
problems for track circuits.

DW down under