London has changed for the better for sure but black snot on the tube hasn't changed at all. I get it from 20 minutes on any deep line like the Victoria. It's from train brake dust, nothing to do with vehicle emissions.
Victoria might be one of the worst for this. You can hear the wheels screaming the abrasion off for long sections of it. It’ll be interesting to see whether the Piccadilly gets better for you when the new trains arrive over the next couple of years.
Because of how old they are there is some really interesting long term data as the earth itself around the tunnels reaches a new thermal equilibrium. In looking for a source, i found a good wikipedia article on tube temp in general.
These comments and the article made me see how much of it is/was due to braking. TIL.
