What's the deal with the grimy, leaky, moss-covered weeping wall at Paddington's Inner Rail platform? It's been doing that ever since I can remember (nearly three years!) and seems to be especially bad after it's been raining.
I reckon the most likely reason is leakage in the rain water gutters which have been seeping for many years through that wall.
There's a number of places like that on the run up and down between HSK and Edgware Road, and at least one of these has scaffolding erected above the track to allow access to it, so I guess that given time this particular one will be addressed to.
Funnily enough only last week I noticed one such wall up there which has been receiving attention, with some brickwork replaced and other parts repointed.
I presume they're doing the work on a 'worst first' basis.