as an alternative to his normal route to the woods , art had kept to the roads instead , walking over the bridge which crossed the disused railway line , the wooden sleepers of which were steadily being reclaimed by the encroaching vegetation , the embankment at the side of the track clogged by nettles and hawthorn bushes , until he had picked up a footpath which ran alongside the back of a large housing estate , before eventually leaving the urban environment behind altogether and emerging into open countryside .