The New York City Subway (or metro, underground railway, choose your favorite name
) third rails are covered on top by a wooden plank that is held several inches above the rail by iron brackets.
The whole assembly (rail and plank) is held about 4 inches above the ties by ceramic insulators.
The primary purpose is to give some semblance of protection to track workers (so they don't step on the live rail while one foot is on the ground). However certain track work calls for covering the entire section of third rail with a thick rubber mat.
On elevated sections, the wooden cover helps prevent snow buildup somewhat.
There are 600 volts across the third rail and ground. I believe it is DC for traction and until recently was generated by rotary AC/DC converters in substations running both at 25 hertz and 60 hertz AC.
The 25 hertz ones have since been taken off line and substituted with solid-state equipment. The train cars each have their own traction motors (modern cars are permanently lashed together in consists of 6), batteries and other things to power the 110-volt lights and fans and heaters inside the cars.
This is what one of those old rotary converters looked like:
and you can read more about them at
]http://www.nycsubway.org/tech/power/sub21-01.jpg [/URL]
Impressive beasts!!!
[This message has been edited by SvenNYC (edited 11-12-2002).]