In message , Christopher A.
Lee writes
No. Current leaking to ground and causing electrolytic corrosion.

Remember, there were already pipes etc under London before the tubes
were built, and the tubes were lined with cast iron segments..

Tram and streetcar track had the running rails at minus 10 volts so
that stray current leaked from ground to the rails instead of vice
versa.

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

Traction current is fed at +420v (positive) on the outside rails and
-210v on the centre rail (negative) the sum of these giving a traction
feed of 630v dc.

They are 'loosely' tied to earth through resistances in the sub stations
that feed the supply. Despite being an electrician in a previous life
and now a driver, I struggle to get my head round 'loosely tied to
earth' and what it means in real life.

I recollect an incident a few years ago when the traction current had
been discharged (turned off) due to person under a train. For
complicated reasons, the centre rail had become live at +420v. The
paramedic who was working under the train and touched that rail and
earth looked extremely pale afterwards. It was described to me like
having your hand hit by a sledgehammer.
--
Steve Fitzgerald has now left the building.
You will find him in London's Docklands, E16, UK
(please use the reply to address for email)