[Boris]

why are some lines eletrified with overhead cables, and others track, whats the advantege of each?

[Camborner]

I'm no expert, but I think it's because overhead electrification allows for faster trains, while the third rail is cheaper.

[poliss]

Very little to do with speed and cost and everything to do with history. To find the answers you have to go back at least 115 years and find out why the Liverpool Overhead Railway chose third rail electrification. http://www.emus.co.uk/zone.htm

[Boris]

thanks, but if it is just history, why has no-one made a hst on 3rd rail, to save costs of building now?
i understand that in history diffrent thing affected it, but why has it not changed today, lots of money could be saved.

[Rog (RJ)]

I think it may be due to the way electricity is able to be "transported" and safety.

Alternating current (AC) can be carried more efficiently at high voltage than it can at lower voltages. This is why the overhead transmission network from power stations to sub stations is at such a high voltage (up to many kilovolts). This allows thinner and therefore cheaper cables to be used. At the main sub stations this high voltage is reduced by transformers to a voltage low enough to be safely carried through underground cables. It then goes to local substations where the voltage is lowered again. When the voltage is lowered the cables have to be thicker to be able to carry the same amount of power (power equals Volt x Amps) The voltage is reduced, so to allow the same amount of power to flow the amperage increases. So it makes sense for overhead lines at very high voltage to be used for the longer distances.

If the very high voltage was used for suburban 3 and 4 rail services, it would be absolutely lethal to go go anywhere near the live rails because the high voltage would be able to flash over. There would also be the problem of insulating the live rails from the ground and the non live rails and preventing flashovers.

Also think about using high voltage overhead equipment in the underground tunnels. The tunnels would have to be much bigger and therefore more expensive to build.

So for practical reasons a lower voltage (although still lethal) of 750 volts for example is used for the suburban services (allowing easier insulation at track level), while long distance lines can use very high voltage (for example 25KV) on the overhead line equipment.

Historically, I think DC was considered safer than AC, although we now know that all forms of electricity (even extra low voltage) can be dangerous if it is not treated with care

This is a simplification and quoted voltages may not be the same as those used in reality

[Boris]

0h, makes sence now, thanks.

[poliss]

I found a webpage that should explain why railways use overhead AC power and not DC these days.
http://www.railway-technical.com/drives.shtml#ACDC

[Rog (RJ)]

I found a webpage that should explain why railways use overhead AC power and not DC these days.
http://www.railway-technical.com/drives.shtml#ACDC

That's an excellent site, thanks for the link