On Sun, 30 Nov 2008 16:00:14 +0000, Graeme Wall
wrote:

In message
David Hansen wrote:

[snip]

Roughly speaking, in a small but complicated system the reduction in
the number of gadgets outweighs the extra rail, but in a less
complicated system over longer distances not having an extra rail is
the important factor. That assumes starting from scratch, but that
is not entirely accurate in a number of ways. Unprotected conductor
rails would not be allowed in a new system anyway and a protected
central conductor rail could probably not be devised,

You could do it providing you never turned the stock, so the Circle line
would be out for a start.

You provide appropriate changeover switches as on the original Great
Northern and City line system which IIRC used two outer conductor
rails until later changed to conventional four-rail working.