Train Electrification System (The Third Rail)

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• published: 23 Feb 2008
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Overview of the third rail system. Podcast for King's College Engineering 2007/08. SCRIPT: Hi, today I'm going to explain how the train electrification system works. Used all around the world, this system allows trains to be powered solely on electricity. In 1890, the world's first electric railway system was opened right here in London. Now, millions of people pass through it everyday yet a small percentage actually know how it works. Let me show you how. Trains, we take one everyday, but seldom do we notice more than the carriage itself. Do you ever wonder where the train gets its power from? Trains run on two main tracks, running parallel to them is a third rail, the source of the train's awesome power. If you look carefully at a wheel and suspension unit of most modern trains, you'll see an additional component, usually located at the first and last section of the train. This is where the electricity is transmitted from the track to the train. The addition is called a sliding shoe and it's held in contact with the third rail at all times. On rail systems like the London Underground, there is a third and a forth rail to close the circuit. Many would ask, "isn't it easier to just use the two existing rails to transmit the electricity?", well, this method is used in many modern trains but it does not function well for large trains as the sleeper, the large rectangular object used as a base for the railway tracks, are not good insulators. Compared to overhead lines, which can sometimes hold voltages of up to 25kV in alternating current, the third rail is charged to a relatively low 600+V by direct current. This restriction is due to the small clearance area around the third rail. This diagram illustrates the three ways in which the shoe comes in contact with the third rail. Namely, the top, side and bottom contacts. The advantages gained from using the third rail systems are: - Lost costs. As third rail systems cost less to install as compared to overhead lines. - Robustness. The third rail is able to take high mechanical forces compared to contact wires or overhead lines. - Easier maintenance access. Since it lies on the ground, it is within easy reach. - Better compatibility. As many railways already utilise the third rail system. and - Visual appeal. There is no need for ugly overhead lines and cables. The disadvantages are: - Less safety for workers. - Limited capacity. Putting high voltages on the rail would cause arcing to occur. - Infrastructure restrictions. Junction require gaps, and though it's not usually a problem, as trains tend to have several contact points with the third rail, trains sometimes get stalled when none of its shoes are in contact with the third rail. and lastly - Insufficient contact caused by fallen leaves, snow, and other debris. And there you have it, the third rail. Hope you enjoyed my podcast, thank you for watching.