Like Mactraxx says - 1000 volts dc, top-running third rail.
Some of the third rail is all aluminum, while most is a composite aluminum (for conductivity) with steel (for rigidity). There's always a few scraps of third rail lying around. If you look from the platforms, you might see some old, worn third rail that hasn't been picked up yet.
There are a couple of spots that are a long way from a substation, which was the justification for the higher voltage. One (Golden Gate Bridge lower deck) was never built, but the transbay tube is the other.
Transbay Tube Third Rail
has some details.