Boltar wrote to uk.transport.london on Mon, 20 Dec 2004:

I had the misfortune of having to use the bakerloo line last
week and noticed, while I was waiting for all eternity at
queens park for a harrow train, that there were 4th rails on
the silverlink only tracks either side that the tube trains don't
use. They looked rather rusty and seemed to head off south towards
euston. How far does it go and what are/were they for? Do tube
trains ever run on that track anymore?

Years ago, when I was a kid, the Bakerloo used to run as far as Watford
Junction, so perhaps those were the tracks you saw?
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/
Website updated 18 December 2004