Steve Dulieu:
The one in the middle is the negative and carries the lower voltage
of 220v, the one furthest from the platform is the positive and
carries the higher voltage of 440v for a combined total of 660v.

Correct. Well, sources disagree on the exact nominal voltage: some
say -210, +420, net 630. The variation from time to time in practice
is likely to be more than a few volts anyway.

The train only sees the voltage difference; it'll run just as well
if one live rail is at 0 and the other is at 630 V. And the power
supply is designed so that this is exactly what happens if a short
develops between one side and the earth. (This is one of the
advantages of using two live rails.)

On tracks designed to be shared with trains from the 3-rail southern
network, like at Richmond, the live rails are always at 0 and +630.

The other two rails (the ones not on insulators) do in fact have
a slight electrical current in them to work the signals ...

Yep.
--
Mark Brader, Toronto | "We are full of digital chain letters and
| warnings about marmalade." --Matt Ridley

My text in this article is in the public domain.