Why does the London Underground have four rails?

Question

Could it not be three, centre for live power and use the runnimg rails for power return

Answer 1

2 are to hold the train wheels these are 4ft 8.5 inches apart.
These are the most solidly constructed with large track ties and a capital I shaped cross section.
The outer one is the positive current usually furthest away from the platform, this has ceramic insulators holding it up.
The central one between the main tracks is the earth, this is needed on an underground train because without it the electricity could find earth by going upwards through the passengers.

Answer 2

The London underground does not want to rely on the electrical capabilities of the running rails which in certain circumstances could become live and fry the travelling public

Answer 3

2 are the running rails 1 is the positive at 420 volts and 1 is the negative at 210 volts

Answer 4

There's no need for return power - all the current (one US trains) is contained in the third rail or the single overhead wire (catenary)... And running rails are energized for signalling at only a few volts...

I believe on the LUG one's positive and one's negative...

Answer 5

Havent got the foggiest, as long as the trains run on time then Im not bothered really

Answer 6

two running rails one positve electric rail one return negitve rail

Answer 7

Three are for the train to run on, the fourth is for people to commit suicide on.

Answer 8

its better than having 3

Answer 9

too dodgy, using the chassis for live current

Answer 10

Could two be spares?