I wonder what line had been the longest out of use but still with track down (excluding quaysides/crossings where it’s not worth taking out).
Track is still down on the few miles running into Portishead, which last had regular freight in 1981 and specials in 1985 for the GWR 150 event. There’s also Leamside (lifted now I think?) and that line in the west mids which I think is being reopened. Any other significant ones?
The only part of the Leamside alignment that has track still on is a a few dozen yards of double track at Pelaw which can be seen below the Tyne & Wear Metro bridge east of the station. The track is disconnected from the Network Rail Heworth to Sunderland line in one direction and I believe peters out round the bend from the erstwhile junction. At all the other locations where roads cross the old alignment, I have not seen any track - indeed in several places (including at Washington near the Nissan Works) the road has been resurfaced and no trace of a crossing is left.
The other end of the Leamside alignment is at Ferryhill - there is no trace of the junction to the mainline between Durham and Darlington.
Of course the Victoria Bridge (viaduct) remains in all its splendour awaiting a dimutive Metro train from South Hylton to the outskirts of Washington to rumble over...
Edit. Both Google and Bing Maps show that track remains on the Leamside line for a significant distance from Pelaw Junction to an industrial estate in NE Washington - but looks as if it has not been used for many years judging by the undergrowth.