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London Underground MoW


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That is pretty cool.


Not only do they got 3rd rail, they got 4th rail too. You think they will ever just use one for power?


- A


I know this is an old post, but I wanted to respond. They will most likely always use a 4th rail on all existing lines. The reason for the 4th rail is the avoid and grounding issues with the large steel tubes that line many of the tunnels. With only a 3rd rail, the other two rails are used as a ground. In the case of using these steel tubes it could cause the ground to come in contact with the steel and corrode it, effectively ruining the tunnel. The 3rd rail runs at about +400VDC and the 4th rail at -200VDC, giving an effective 800VDC current through the traction motors and keeping any electricity from going to the steel tubes used to line the tunnels.

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