The Natural Philosopher wrote:
On 01/04/2021 16:06, Andy Burns wrote:
NY wrote:

I've always wondered why the two rails are not symmetrical about 0V -
ie +315 V and -315 V rather than +420 V and - 210 V.

"The positive rail is 3 inches higher than the running rails, while the
negative rail is 1.5 inches higher.Â* The positive insulators are thus
twice the height of the negative ones and therefore have about twice the
earth leakage resistance, so the voltages are set with a proportional
disparity between the positive and negative voltage levels.Â* The
positive rail is at a potential of 420 volts above earth and the
negative rail at 210 volts below earth."

http://www.trainweb.org/tubeprune/tractioncurr.htm

changed since my day then, when underground ran on SR three rail
systems quite happily...and could use either centre or side rail

You have a misunderstanding somewhere.
No either about it , Not now and not since they were electrified over a
100 years ago whatever your day was when you made a wrong assumption.
Underground passenger trains *, have to be in contact with both side and
centre rails
to complete their circuit, there is no path built in via the axles and
wheels with brush gear to by pass axle bearings so they dont get damaged
by high currents passing through them like 3rd rail ( and overhead equipped
) trains have.
What happens on the shared sections is the outer rail voltage is raised to
the full potential required , once 630 but often now the later 750 .the
centre rail is simply bonded to the running rails and is at ground
potential. The observant who used the shared sections around around Putney
may have noticed at one time to save money the centre rail was sometimes
mounted on wooden blocks rather than insulators. Special arrangement have
to be in place where the two systems meet so a train doesnt bridge both
system so there are long gaps in the live rails on Putney Bridge and
between Turnham Green and Gunnesbury. Photos of the Putney bridge ones and
a drivers description on this link.
http://www.trainweb.org/districtdave...ey_bridge.html

* some of the maintenance locos are switchable in case they get contract
work on the third rail system.

We are getting away from DIY but the technically nosy may find a detailed
description of how such systems in the UK were developed here.

http://www.clag.org.uk/3rd-4th.html


GH