http://www.nytimes.com/2002/07/30/nyregion/30TUNN.html
"But now, harnessing the mass and momentum of the new train cars,
the subway's electricians are trying to strike up a better
relationship between train and rail. In theory, it works like
this: A moving train consumes power. When it stops, however, it
can use its motor as a generator and pump some of that power back
into the third rail, to be consumed by other trains around it."