On Sun, 30 Nov 2008 12:38:22 -0000, "John Rowland"
wrote:

MIG wrote:
On Nov 30, 11:14 am, David Hansen
wrote:
On Sat, 29 Nov 2008 09:26:38 -0800 (PST) someone who may be MIG
wrote this:-

LU's centre rail is I believe at minus 200 volts.

That's interesting, because I was under the impression that there
was no danger from the middle track,

A dangerous impression. If there was no (electrical) danger from the
central conductor rail then it would not be mounted on insulators.

Insulators are a give-away that something is energised.

Piecing it all together, I suppose it adds up, in that if a neutral
rail was to be used, it might as well the the running rails or
something earthed. So the fact that there's a special rail on
insulators means it can't be neutral. Never really thought it through
though.

No, the fourth rail was used because allowing the current to return though
earthed running rails causes corrosion to any metal utility pipes in the
area, so the fourth rail and insulators are there to protect water mains.

On a related note, the original LNWR/LMS electrification was 4th rail.
It was converted to three with the return and running rails bonded, in
1970.

Of course it made sense to use the existing 4 rail system because
apart from the LBSCR's overhead AC that was what the other London area
lines used. The LNWR electrification was planned in 1907 and opened in
1914, with the Bakerloo linking up a year later.

The LSWR 3 rail system was planned later with the first section
opening in 1915.