It's on one side OR the other, depends - switches back and fort (there is no center rail) - it looks different, and usually has a wood/fibreglass cover over it (up about 4" or so) - it's WIDER than a regular rail too

In the city subway, it's 600 Volts (660 actually - I think) and at least used to be 25Hz (think it still is)

Interestingly - the communter trains around here are also 3rd rail - Now the LIRR has "top shoe" 3rd rail - like the subways - aka the "shoe" (which is on both sides of the car - and BTW both sides are hot - they are connected together) rides on TOP of the 3rd rail, but Metro North has Bottom Running 3rd rail - the rail is suspended, and the shoe slides on the BOTTOM of the 3rd rail - each as it's advantages

if you look at this photo - the 3rd rail is up aginst the wall (in stations, they try to put the rail away from the platform, so that in case someone does fall, and someone tries to pull them up, there is no 3rd rail to step on)

http://www.nycsubway.org/perl/show?54488