An electric locomotive is a locomotive powered by electric motors which draws current from an overhead wire, a third rail, or an on-board storage device such as a battery or a flywheel energy storage system.

The first known electric locomotive was built by a Scotsman, Robert Davidson of Aberdeen in 1837 and was powered by galvanic cells.

Davidson later built a larger locomotive named Galvani which was shown at the Royal Scottish Society of Arts Exhibition in 1841.

Modern electric locomotives range from small battery-powered machines for use in mines to large main-line locomotives of 6,000 horsepower (4.5 MW) or more.

In reality most modern locomotives are electrically driven. Pure electric locomotives take their electrical supply from an external source while diesel-electric locomotives carry their own generating station.

Main line electric locomotives first appeared at the beginning of the 20th century. The reason for their introduction was the problem of smoke, especially in tunnels. In the UK this was the London underground system while in the USA it was under river tunnels and the need to eliminate smoke in built up areas.

Early electric locomotives all relied on external power sourcing. Once up and running they tend to be reliable and efficient, but the supply infrastructure is a large capital expense that does require ongoing maintenance. For this reason only heavily used lines could justify electrification. For suburban lines the reduction in pollution from steam locomotives was a benefit all were aware of.

Alternating current or direct current?

Early locomotives came in a variety of forms. Generally they were designed to run off the supplied current, so locomotives with a direct current (DC) supply had DC motors while alternating current (AC) supplied locomotives with AC motors. AC can be either single or three phase. While the former requires two wire supply, one overhead the other being the track, three phase requires three supply wires. Three phase locomotives therefore had two overhead supplies, the track being the third.

DC supplies were either overhead or by means of a track level supply commonly called the third rail.

AC traction motors tended to be larger than DC motors. This often meant electric locomotives with steam engine type cranks. DC motors could be smaller and set up to drive the axles, usually through a gear, but in some early examples by being part of the axle. Even so, some notable DC electric locomotives had large DC motors driving large driving wheels.

One possibility with electric locomotives is that the motor can be used a generator during braking, feeding electricity back into the supply system. Generally called regenerative braking. This is not a new idea, it was one reason for the adoption by some railways of 3 phase AC supplies. Especially in mountainous areas where the locomotive going down would generate much of the supply for a locomotive going up. Italian railways used this system in the Alps.

Today all electric locomotives tend to have drive motors close to the axles, although some still have the motor in the body driving the wheels through internal drive shafts.

Modern solid state electrical control systems means the motor does not need to match the supply. This means multi-voltage cross border locomotives are now quite common. Drive motors are generally DC, but there are 3 phase drive motors on some locomotives.

See also

• Locomotive
• Railway electrification system