Quote:
Originally Posted by JamesPB
First of all I didn't realise that any of the mainline railways had track-source power. I thought all mainline electircs in the UK were the same as in sweden i.e. overhead cables connecting to pantagraphs.
If that is not the case, is the network rail system the same as the Underground's?............................
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James,
Many Network Rail lines in London and the South-East and on Merseyside use a system where power is collected from a live third rail. The return current then passes via the normal running rails and some of it leaks into the ground. The London Underground system is very similar but uses a live third
and fourth rail. Having the fourth rail means that the normal running rails do not have to be used for the return current so preventing most of the leakage to earth. Current leaking to earth on Underground lines built inside cast iron tunnels causes corrosion so must be avoided.
As Boilersuit says, all trains have more than one set of current collector shoes so gaps in the live rails at level crossings and pointwork do not pose a problem. As you pointed out in your initial post, level crossings are rare on the surface lines of the London Underground system but they are very common on the third rail lines operated by Network Rail.