On Sep 21, 11:08*pm, Charles Ellson
wrote:
On Tue, 21 Sep 2010 10:15:19 -0700 (PDT), The Gardener wrote:

AIUI, only the Euston - Watford DC line

And NLL (where still DC) and WLL ?

uses only one running rail for
traction return and this is why the redundant fourth rail remains
north of Harrow and Wealdstone; it is bonded to the return rail to
reduce the return circuit resistance.

IMU rather to prevent increasing it IYSWIM (see also "chicken and
egg") as it was originally designed as 4-rail with substations spaced
appropriately.

Agreed!

Apologies for not responding to this one sooner: to confirm, the WLL
is electrified on Southern principles and uses both running rails for
traction return. The sole surviving DC section of the NLL (Acton
Central to Gunnersbury Junction) uses one rail. One of the odd
consequences of the re-electrification of the NLL was that the DC
section to Gunnersbury is now isolated. A new twin-transformer/
rectifier substation had to be built at Acton Central as the original
sub only had one transformer/rectifier unit, and there would then have
been no contingency if that had failed. The logical (ISTM) solution of
putting in a new sub in the Gunnersbury area and supplying it from the
Southern's 33 kV distribution system was not, for some reason,
considered. As a result, an 11 kV feeder runs from Acton Lane solely
to supply Acton Central.