overground trains

[filanto]

what is the third rail for on overground railway tracks?

[Stav's Back]

Where the trains are powered by electricity, like they are down here in kent, the third rail is the "electric rail", providing the power for the train. Overhead cables are sometimes used instead. If a train is diesel, then there would be no third rail.

[Toffee]

Why, on the Tube, do they need four rails?

[pinus]

The first tube railways - City & South London (now part of the Northern Line) and the Central London Railway (now the core of the Central Line) - used a three-rail or earth return system. In 1904 - 06 the District, Metropolitan and new tube railways introduced the four-rail system in use today, with separate positive and negative rails. This was introduced to overcome fears of corrosion of underground pipes and cables which could result from using a three-rail system The negative rail is in the centre with the positive rail on the outside. Current is supplied at 630 volts DC. Where Underground services run over ordinary three-rail tracks (north of Queens Park and on the Wimbledon and Richmond branches) a negative rail is provided to allow the Underground trains to run; that rail is bonded to the running rails to allow current return for both types of train Info from London Underground Official Handbook

[Toffee]

Thanks pinus. Does that mean that on a three rail system the return is "returned" through one of the wheel rails or is it run to earth or more probably run staight back beneath the earth to complete the circuit? Does that question make sense? And if it is returned down a "wheel rail" does that means that one of the "wheel rails" is also live? I can't believe that it is. But I seem to recall in the dim and distant past that such a system might have been used on domestic model railways.