As Jeff Goldblum kept saying in Jurassic Park, nature finds a way. We're metres underground, in the artificially lit, hostile environment of Liverpool Central. Trains are whizzing past every five minutes on electrified tracks. There's hardly any fresh air. But somehow, these plants have managed to find a little dirt, a little water, and have started to grow out of the drainage holes by the track. It's almost inspiring, even if it does raise troubling questions about the maintenance regime.
(Also, has the Doctor dropped his sonic screwdriver under the tracks, or what?)