On Thu, 17 Sep 2009 01:55:45 -0700 (PDT), Sim wrote:

Some differences between Overground and Underground:
1. Third rail electrification rather than fourth, so not compatible
for through running. Choosing fourth rail would have been (a) very
expensive with no obvious gain [why convert Willesden Junction--
Clapham Junction for example, or indeed the North London?] (b) The
reason for fourth rail on the Underground is the need to run through
metal tubes, which give rise to induction and other problems with
running-rail-return systems (they were tried, and abandoned). No metal
tunnels on Overground routes: the ELL is masonry, of course.

How much of the Met/Circle/District/H&C lines run through metal
tunnel? Why aren't these switched to the 3-rail system?