In reality most modern locomotives are electricaly 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
tunnlels caused by steam locomotives. In the UK this was the London underground
system while in the USA, it was under river tunnles and needs 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 efficint, 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 ofl
The world speed record for a wheeled train was set in 1990 ba a French TGV
which reached a speed of 515.3km/h (320mph).
While recently designed electrififed railway systems invariably operate on
alternating current, many existing direct current sytems are still in use –e.g. in South
Africa,Spain,and the United Kingdom(750v and 1500v); Netherlands(1500v); Belgiu,
Italy, Poland (3000 v), and the cites of Mumbia and Chicagio (which will be switched
to AC by 2025).
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 a alterntating current(AC) supplied locomotives with AC motors. AC
can be either single or three phase. While the former requies two wire supply, one
overhead the other being the track, three phase require three supply wire.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 smaller 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 as a
generator during braking, feeding electricity back into the supply system; this is called
regenerative barking. This is not a new idea, it was one reason for the adoption by
some railways of 3 phase AC suppies. Especially in mountainous aresa where the
locomotive going down would generate much of the suppy for a locomotive going up.
The Swiss railway uses the system; three modern locomotives heading downwards
generate enough power to power a single locomotive in its upward journey.
Today all eclctric 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.