On Sun, 30 Nov 2008 03:46:02 -0800 (PST) someone who may be MIG
wrote this:-

Piecing it all together, I suppose it adds up, in that if a neutral
rail was to be used, it might as well the the running rails or
something earthed. So the fact that there's a special rail on
insulators means it can't be neutral. Never really thought it through
though.

The reasons for adopting the four rail system remain sound. As well
as corrosion it allows a simpler arrangement for the traction and
other electrical systems, like signalling. With a three rail system
one needs gadgets like impedance bonds to keep the electrical
systems separate enough to avoid interference, but there is one less
rail to install and maintain.

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, but protected
conductor rails could be fitted either side of the running rails.

None of that outweighs the fact that low voltage conductor rails,
especially unprotected ones, are not ideal and were things being
done from scratch a high voltage overhead system, with the larger
tunnels this implies, would be chosen. In fact an overhead system
wouldn't in fact involve any larger tunnels. As any new system would
be fitted with emergency walkways and so the tunnels would be larger
anyway.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54