Electric trains use electricity to power electric motors, driving their wheels and providing locomotion. The electricity comes from one of three sources. It is either delivered via an electrified third rail beneath the train, via electrified cables above the train, or stored in batteries on board the train.
Electric trains have a long history, and they have advanced greatly during that time. Modern electric trains commonly use both batteries and an external power source for reliable power. One advantage of having a battery, particularly relative to alternative technologies such as diesel trains, is that batteries can be charged via regenerative braking, making this much more efficient for commuter rails and similar systems that make many starts and stops. Many modern diesel trains are hybrids, with batteries supplementing their internal combustion engines for just this benefit.
Power delivered by a third rail is, in some ways, the simpler of the two external power sources. While each stretch of powered rail needs its own transformers, the trains use the direct current from them. The systems are cheaper to build and require little maintenance. However, they are not suitable for streetcars or other situations where foot traffic could bring pedestrians in contact with the rail. The alternating current cable systems are suitable for these situations, but are more expensive and require onboard electrical transformers that require more maintenance and often produce interference with electronic communications in their areas.