On Tue, 25 Nov 2008 08:57:17 +0000, David Hansen
wrote:

On Mon, 24 Nov 2008 20:32:20 -0000 someone who may be
wrote this:-

Is a fourth rail really necessary out that way, however? I always understood
that they were really needed only for the tub sections of the tube, to help
power flow into the motors. I heard this on the Island Line, where 38 stock
operates only with a 3rd rail.

It operates there after being modified to allow the traction supply
to travel via the running rails. Heavy electric currents through
bearings is not a good combination, hence the modification.

There was IMU some trouble of that nature when c.501s were converted
from 4-rail to 3-rail in the early 1970s.