On Feb 18, 8:51*pm, wrote:
On Wed, 16 Feb 2011 19:18:44 +0000
"Clive D. W. Feather" wrote:

More precisely, what I'm saying is that you can't just stick down some
third rail and say "that's it". You need to modify the existing AC
electrification to be compatible with dual-voltage. And, since the ECML
is currently not signalled with DC-immune equipment, you'd need to
replace it.

I wonder if its possible to have a signalling system that doesn't use the
rails at all so avoiding all of these issues. Perhaps using magnetic
detectors or infra red beams or suchlike?

Yes, it's possible, and will happen. The trouble is, FAILSAFE.

ETCS, which the UK's railways are gradually migrating to, is precisely
such a system, using RF transponders mounted on the train and between
the sleepers, but since a) it involves replacing all of the signalling
infrastructure that we have; b) railways have to be extremely safe;
and c) as Clive says, unexpected changes in practice can have massive
unexpected negative consequences, it's going to take a while to bring
it in.

--
John Band
john at johnband dot org
www.johnband.org