On Tue, 13 Apr 2010 22:00:22 +0100, tony sayer
wrote:

The WCML modernisation scheme uses (mostly!) 400 V three-phase, but
this has not been a great success owing to the higher currents (and
therefore greater number of sources of supply, known as Principal
Supply Points or PSPs) required.

Ummm.. Rather interesting this but this 400 volt 3 phase supply was
"difficult" in quite what way for signalling purposes?..

As implied above, it requires more transformation points and/or more
copper in the ground. If it replaces a single-phase supply, it also
(assuming 3P+N) doubles the number of wires to be played with (or
stolen) and enables the various "interesting" effects caused by breaks
in the neutral or one phase wire without other conductors being
interrupted.