User blog comment:Stormwell/Thoughts on Technology/@comment-1500935-20140210135550/@comment-3525200-20140213005820

Yeah, I know it seems weird but the London Underground uses four rails (though that works in a different).

I hazard a guess that the four rails is mainly for the electrical pickup of the vehicles, one set of wheels picks up the current whilst the other returns it. Or basically positive set of wheels and negative set of wheels.