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


--
"When a true genius appears in the world, you may know him by this sign,
that the dunces are all in confederacy against him."

Jonathan Swift.