In the London "Underground" there is a rail that runs right between the two running rails. (There is you regular "3rd" rail also.
So: what's the story with the London system. Why do they do it the way they do and why does NYC do it the way it does?
(I asked this question on the NYC. Transit group. One guy said that the center rail was -125 vdc in the tubes and is ground potential on the above ground portions of the line. The 3rd rail is positive and provides the rest of the total 600+ volts needed to operate the train. The reason for the 4th rail was said to be certain unstated problems with "leakage.")