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.

OK what sort of current draw does a typical signal post, gantry frame,
need then?...

The relevant figure is probably something more like how much average
current is drawn overall by the various equipment in the general area
x miles away at the end of the cable, remembering that other than
signal lamps and track circuits ...

Yes.. Just wondered if that had now fallen due to LED lighting etc?..

--
Tony Sayer