18 years ago
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.")