On Mon, 21 Sep 2009 15:27:49 -0700 (PDT), MIG
wrote:

On 21 Sep, 23:08, asdf wrote:
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?

LU power supply, stock movements, shared sections etc?

On the sub-surface lines there are still armoured cables (and in the
old days, lead-sheathed cables) and air pipes which don't take kindly
to traction currents taking a short cut through them. More modern
materials and methods possibly reduce the risk of stray currents but
the signalling systems in current use IMU are still designed around
running rails devoid of traction currents.