why do subway trains need only one electrified third rail? I thought electricity needs a negative and positive? shouldn’t we need two electrified rails?

Question

why do subway trains need only one electrified third rail? I thought electricity needs a negative and positive? shouldn’t we need two electrified rails?

In: 2

Answer 1

Some systems do – the London Underground for example has 2 electrified rails.

Generally though negative current is handled through the 2 running rails. A complexity is current leakage from these which can be problematic in built up areas ( why they adopted 4 rails for London in the 1880s) which is now handled by sophisticated insulation between the running rails and the support.