"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?

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?