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?
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On the London Underground, there are indeed four rails: two running rails; a positive rail; a negative rail.
For systems which have only three rails, both the running rails are at a notional ‘ground’. The train runs on the potential difference between that ground and the third rail.
On four rail systems, the running rails are still usually bonded to ground, but do not form part of the circuit. Also, the two current rails are asymmetric about ground. LU uses 630V DC, with the +ve rail at around 210v and the negative rail about -420V. Or maybe is the other way round. I can’t remember.
The reason they do this is to avoid weird electrolysis effects with the metal tunnel linings.
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