On Sun, 21 Jun 2015 02:07:28 +0000 (UTC), Recliner
wrote:

Basil Jet wrote:
On 2015\06\21 03:01, Recliner wrote:
Charles Ellson wrote:
On Sun, 21 Jun 2015 02:41:35 +0100, Basil Jet
wrote:
IIRC Drayton Park has both fourth rail and overhead,
although obviously not for much distance.

The fourth rail might not be doing a lot unless it has been left
connected but dropped onto the sleepers as done between Harrow and
Watford.

It's certainly not a functional fourth rail, and nor would 313s connect to
a centre rail anyway.

So why aren't all of the water pipes near the GN&C corroding merrily,
which is the problem 4th rail was invented to solve?

Probably nothing more than a lack of leakage current. If the tunnels
are fairly dry and the traction bonds are in good condition then the
leakage will be minimal. Many of the water pipes "upstairs" will now
be plastic or interrupted by plastic sections which will further limit
the flow of leakage currents through them; they're only going to
corrode "merrily" when you have a combination of water and metal
touching a different metal or other conducting material. On various LU
lines, the only reason for 4-rail electrification is for
standardisation.

There's plenty of third rail track in tunnels around the world and, indeed,
in London. Just think of the ELL.