The "Third Rail" in the NYC Subway system is a rail carrying 600 Volts, DC. The rail can be on either side of the main tracks, as there are pickup "shoes" as they are called on both sides of the train.

The main tracks are also a conductor, serving as the "ground" to the 3rd rails "hot" (think home wiring - the third rail is the black wire, the tracks are the white wire).

Here's an illustration I made to show how it works:



The current from the third rail powers all functions of the train.

The third rail itself is only slightly guarded from accidental contact through the installation of a wooden insulating plank mounted above the rail.

This system is also used by the Long Island Rail Road and hundreds of other systems.

see more: http://en.wikipedia.org/wiki/3rd_rail