Direct current traction

Railway electric traction describes the various types of locomotive and multiple units that are used on electrification systems around the world.

Railway electrification as a means of traction emerged at the end of the nineteenth century, although experiments in electric rail have been traced back to the mid-nineteenth century.Thomas Davenport, in Brandon, Vermont, erected a circular model railroad on which ran battery-powered locomotives (or locomotives running on battery-powered rails) in 1834.Robert Davidson, of Aberdeen, Scotland, created an electric locomotive in 1839 and ran it on the Edinburgh-Glasgow railway at 4 miles per hour. The earliest electric locomotives tended to be battery-powered. In 1880, Thomas Edison built a small electrical railway, using a dynamo as the motor and the rails as the current-carrying medium. The electric current flowed through the metal rim of otherwise wooden wheels, being picked up via contact brushes.

Electrical traction offered several benefits over the then predominant steam traction, particularly in respect of its quick acceleration (ideal for urban (metro) and suburban (commuter) services) and power (ideal for heavy freight trains through mountainous/hilly sections). A plethora of systems emerged in the first twenty years of the twentieth century.

Direct current (DC) traction units use direct current drawn from either a conductor rail or an overhead line. AC voltage is converted into dc voltage by using rectifier.

All alternating current (AC) Traction units draw alternating current from an overhead line.

Because of the variety of railway electrification systems, which can vary even within a country, trains often have to pass from one system to another. One way to accomplish this is by changing locomotives at the switching stations. These stations have overhead wires that can be switched from one voltage to another and so the train arrives with one locomotive and then departs with another. The switching stations have very sophisticated components and they are very expensive.

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