Wow, check these things out:

http://www.youtube.com/watch?v=KGb-nUK41tc
http://www.youtube.com/watch?v=UmM68nnP1CU
http://www.youtube.com/watch?v=lNAKL9qtnIA

A very cool way to get DC. Kind of like an extreme mercury lamp. Some of these things, still in use, are over 70 years old and are original!

Yep, these were huge step forward in electrical power conversion technology. They reduced converter's losses by more then ten times compare to older mechanical converter, mainly at light load
 and made the converter lightweight and small, so e.g. possible to fit into train.
So they allow the use of electricity as main power even for long distance railways - it allow to use high voltage power grid frequency (e.g. 25kV) for supply and transmission (so simple low loss transformers are enough for feeding points along the railway) and at the same time convenient 1..3kV for control and motors (so no arcing problems).

Of course, these bulbs are replaced by semiconductor rectifiers, but then the further energy saving gain is minor, the driving force was only the compactness and mechanical robustness of semiconductor devices, the train power usage efficacy lies mostly on skills of the train operator...