For electric engines to work they need access to a power supply. There are two main ways of supplying power, by means of an overhead wire or by means of a third electric rail.
The overhead system has suspended
wires all down the length of the railways. There is a pick up
arm called a pandtograph on top of the engines, which make contact
with the overhead wires allowing transfer of electricity. The
Standard in France and the UK for power supply is 25kV AC fluctuating
at 50Hz although other countries may use a lot lower voltages
like 3kV
When electricity flows across long wires some of the power is lost in transporting it. The amount of power efficiency is R/I2 (R is resistance, I is current). So clearly for most energy efficiency it is best to keep the current as low as possible. However the train will need a certain amount of power. Power is equal to the voltage multiplied by the current. So if the voltage is very high, the current can be very low, and the system remains efficient. For the same reason, in powerlines carrying electricity across the country the voltage is as high as 400kV, or 400,000V
The main disadvantage is that electricity can jump through the air. For every 1kV or 1000V electricity can jump about 1 centimetre. So 25kV can jump 25cm, or about ten inches. This means for powerlines running at 25kV, everything connected to the ground has to be kept at least that distance away, and this can be problematic, especially when it comes to tunnels and bridges. Also, all the supports for the over head cables have to have a piece of insulation at least 25 cm thick. This is why some have chosen for a lower voltage.
The
idea is simple, an extra steel rail is laid along side the track,
and its primary purpose is to carry electricity. Then on the trains
a pick-up shoe sits on the rail which provides electricity to
the train. The 3rd rail sits slightly higher up than
the other rails, and instead of being supported by sleepers, it
is supported by a series of insulating supports.
Britain is the only country in the world with extensive 3rd rail electrification of main railways. The reason for this was because back in the early part of this century, Southern Railways, who owned the railways in the south east of england launched a program to get rid of the steam engines. They decided to electrify all their lines. Now of course back in those days no one had established the overhead wires system, the only example was the London Underground which used electicity carrying rails so Southern adopted that idea. So today we have third rail electric systems running at 750 V.
The
London Underground uses two electric rails, a positive and a negative
direct current at 660 V, so it in fact has four rails. Most urban
transport trains use a 3rd rail or a 3rd
& 4th rail system
The energy efficiency of transporting electricity is as mentioned before R/I2 (R is resistance, I is current) so it is always desirable to keep current low. However in the 3rd rail system the cross sectional area of the rail is much greater than a cable, this means the resistance is very much lower, so the need to keep the voltage high is generally not so important. Being so close to the ground if the voltage was too high it would be very hard to insulate the rails and could lead to further energy losses.
It is very cheap relatively speaking. Instead of having to construct a whole series of supports with wire suspended under high tension, all that's needed is to lay an extra railway down. Also when railways go through tunnels it is very costly to build overhead lines as it often requires the roof of the tunnel to be raised, and the same goes for bridges.
There are a number of disadvantages for using 3rd rail. Power is a key issue, for light local passenger trains its fine, but cannot supply enough power for really heavy freight trains. Secondly speed, the 3rd rail is not continuous, it has to break for points and crossovers and sometimes to switch from one side to the other. Trains have to slow so that damage isn't caused to the shoe. Also 3rd rail trains are all limited to 100mph or 160km/h, not suitable for high speed. The class 73 is the only 3rd rail locomotive in the UK, and if you look at the locomotive list you can see that it is significantly less powerful than other electric's. Mostly electrical multiple units use the 3rd rail system.
It may be cheap, but the problem is because of lack of power and speed, it doesn't offer much advantages over diesel trains, which is why electrification would be considered in the first place. However for underground railway systems it has caught on, clearly diesels cannot run in closed tunnels without causing massive health problems, and because the tunnels in underground are generally small, there isn't room in the roof for overhead lines, so electric rails are ideal.
See Also:
Electric Trains
Electrical Multiple Units (EMUs)
EMUs operating in the UK
Locomotives Operating in the UK