I recall working in central London in the 90s and having black snot all the time. Where is it now on the snottogram-scale?
That said it is much better these days than it was perhaps 8-10 years ago in my experience. TfL have started doing some more involved cleaning at night I think - something like magnetic dusting and regular industrial vacuuming to try and control the dust.
The tunnels are also regularly cleaned; I doubt there's much if any Victorian steam train grot still floating around.