Use of 4 rails on London's Northern Line

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Use of 4 rails on London's Northern Line

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1ulmannc
Modificato: Feb 8, 2014, 3:56pm

The latest copy of Today's Railways UK just arrived here in the US about a week ago or so. The photos of the Northern Line show 4 rails for each path. Why 4? Is there a book or link that explains this?

2thorold
Feb 8, 2014, 4:08pm

http://en.wikipedia.org/wiki/Railway_electrification_system#Fourth_rail

Two rails for the wheels, two for the electric current.

In the early days they had trouble with electrolytic corrosion caused by the return current through the running rails and metal tunnel linings, so they added the negative centre rail to eliminate the need to run current through the running rails.