On Sun, 30 Nov 2008 15:08:16 +0000 someone who may be Graham Murray
wrote this:-

Insulators are a give-away that something is energised.

Does that necessarily follow?

Pretty much.

If the reason for using a 4-rail system
rather than a 3-rail with return via the running rails were to avoid the
problems such as electrolysis and interaction with signalling, would it
not be possible to do it by having the centre insulated rail at a
nominal ground potential but only bonding it to ground at the
substations?

That would mean, for the same voltage difference between the
conductor rails, increasing the voltage on the other conductor rail
and thus needing larger insulators for that. One would also have
greater leakages, as the higher the voltage the greater the leakage
through something like damp ballast against the conductor rail.

However, one would still be able to use heated conductor rails,
which is not AFAIAA done (at least in the UK) as that would also
involve heating the running rails (amongst other undesirable
things).


--
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