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?

B2003