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. 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. 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