On Sat, 29 Nov 2008 14:06:54 -0000, wrote:

"Charles Ellson" wrote in message
.. .
On Mon, 24 Nov 2008 20:32:20 -0000, wrote:

Is a fourth rail really necessary out that way, however?

It is if you want the same train to work both there and on the
Underground.

Was the 38 stock on Island Line modified to work only on third rail? What
did that require, if that was the case?

I always understood
that they were really needed only for the tub sections of the tube, to
help
power flow into the motors.

No the LU 4-rail power supply is intended to keep the traction current
within the two conductor rails and not find its way back via other
bits of metal with consequent damage.

In case of flooding, perhaps?

No. Current leaking to ground and causing electrolytic corrosion.

Remember, there were already pipes etc under London before the tubes
were built, and the tubes were lined with cast iron segments..

Tram and streetcar track had the running rails at minus 10 volts so
that stray current leaked from ground to the rails instead of vice
versa.

LU's centre rail is I believe at minus 200 volts.

This is a major problem for the elevated sections of the New York
Subway, which is 600v 3-rail running on continuous girder bridge. I
saw some TV footage of the electrolyic corrosion damage recently.

I heard this on the Island Line, where 38 stock operates only with a 3rd
rail.

No tube tunnels or aforementioned metalwork to worry about.

Then what is the problem with Bakerloo line trains continuing further north,
assuming that they don't require any modifications?

They would need them - and would probably be prohibited from the
underground section.