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Charles Ellson[_2_] · November 27th 20, 12:37 AM posted to uk.railway,uk.transport.london

On Thu, 26 Nov 2020 11:28:09 +0000 (UTC),
wrote:

On 26 Nov 2020 10:33:55 GMT
Marland wrote:
On sections shared with third rail trains the positive rail has always
been at the full potential whether
that has been 630V or 750, the fourth rail becoming neutral like the
running rails to which it is bonded. I don’t know what the arrangements

I can't seem to find a definative answer about that line, but it would
seem that you're right about thr 4th rail being bonded to the running rails
and the way tube trains can run on the Watford line is that the 3rd rail
was only at 630V so mainline trains were running on a lower voltage rather
than tube trains running on a higher one which makes more sense. I'd guess
the Richmond and Wimbledon routes are the same. Or were until the S stock
came along.

Crossposted to utl.

On the DC line the 4th rail is bonded to one running rail, usually the
one adjacent to the 3rd rail. That rail has traction bonds on it while
the other running rail only has track circuit bonds. As for borders
with other voltages, there is no special isolation at Queens Park; the
two systems appear to be able to cope with the relatively short period
of bridging if in fact there is any in the absence of a traction bus
on LU stock, only LU trains can span the two systems. The other
borders (Watford, Euston, Willesden) are with AC electrification where
bridging does not arise unless something is seriously wrong with an
EMU's electrics.