A third rail is an electric rail installed in subway and some light rail systems to provide power to electric trains. Many subway systems use this method of power supply because it is cheap to install and runs relatively efficiently, although there are safety problems associated with it. Many consumers who use subway systems are familiar with the dangers of the third rail, since they are usually clearly marked. These well publicized dangers have led to the use of the term in slang in reference to a political issue that is so charged that it is essentially untouchable.
Early subway systems used combustion to power their trains. This generated a great deal of pollution, making the air difficult and unpleasant to breathe. Many switched to electricity as soon as it became available, because it was so much cleaner. There are a number of ways to provide electricity to an electric railway, including using overhead wires and installing a third rail.
The third rail lies on the ground, next to the tracks used by the train. Depending on the system, it may be located to one side or in the middle of the tracks. A paddle extends from the train to come into contact with the rail, conducting electricity to the train to power it. Many trains can use the system at once, disengaging when they no longer require power.
The clearest disadvantage to the third rail is that it can be very dangerous. It constantly carries a very high electrical current, which can be fatal to animals and humans that come into contact with it. Many safety systems are aimed at preventing contact with the rail, ranging from sheaths that make physical contact difficult to high platforms and guard rails meant to make it more difficult to get onto the tracks.
In areas where tracks are level with the ground, however, there have been incidences of severe injury and death as a result of contact with the third rail. On standard elevated and underground tracks, the rail is usually located on the side of the tracks opposite emergency tunnels and stairways, to ensure that people will not come into contact with it if they need to evacuate the train. In stations, the rail is located against the wall of the station, rather than on the platform side, so that people who fall onto the tracks will be in less danger.