A third rail is a metal rail that is placed on either side of a train track to provide electrical power to trains traveling along the tracks. They are often used on subways and local train services. Before third rails, electricity used to be carried in overhead wires.
Electricity as a method of giving power to a train came about during the 1800s. Scottish engineer Robert Davidson designed and built an electric locomotive in 1837 using batteries. Using batteries meant that the locomotive could not be used on the main railways due to the constant need to charge them. It was not until German engineer Werner von Siemens in 1879 that the first design of a reliable electric locomotive to carry passengers was made. Siemens' design used a 2.2kW electric motor and had a top speed of just over 13km/h. This new locomotive hauled three passenger cars along a 300 meter circular railway track and carried around 90,000 people in four months during its trail run at the Berlin Industrial Exhibition. Siemens' electric locomotive was the first train to utilize a third rail and power was provided via a dedicated dynamo. The early designs of electric trains were unable to access a widely available and reliable source of electricity. A network of electricity that went where ever the network of rail went was needed. Whilst many modern distance trains utilize an overhead wire system to provide power, the third rail was the first viable means of supplying power and is one that remains very much in use world-wide today.
Technical aspects [change]
The third rail is usually located outside the two running rails, but occasionally between them. The electricity is transmitted to the train by means of a sliding shoe, which is held in contact with the rail. On many systems an insulating cover is provided above the third rail to protect employees working near the track; sometimes the shoe is designed to contact the side (called side running) or bottom (called bottom running) of the third rail, allowing the protective cover to be mounted directly to its top surface. When the shoe slides on top, it is referred to as top running. When the shoe slides on the bottom it is not affected by the build-up of snow or leaves.
Third rails typically carry hundreds of volts in high amperage current. This makes them very dangerous. In 2005 American James Maki fell onto the third rail at a Boston rail station and suffered extensive and disfiguring injuries. As part of his treatment he became the second American recipient of a face transplant. Many deaths occur around the world each year due to human contact with a third rail. In New York City there have been multiple cases of people dying because they have urinated on the third rail.
- American Institute of Electrical Engineers (1921). Transactions of the American Institute of Electrical Engineers. American Institute of Electrical Engineers. p. p. 1625.
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