Strange...about 60% of the London Underground is above ground. To ameliorate the 'leaves on line' problems, each line has a couple of RATs, or Rail Adhesion Trains. These are usually modified last generation units.
They don't seem to have nearly the grief that Network Rail has, although the RAT to track mileage ratio is a good deal better for LU.
Also less problems with trees near tracks, being so urban and long standing.
I’m the days of steam, there were fewer line-side trees, due to fires started from hot ashes. It had been over 60 years since the end of steam, and so a lot of resistance to clearing these trees and reducing the risks.
They’ve got sanding on most trains now, and a set of water-jetting and gluey-grit applying machines, but it needs just one gust, followed by a regular train at a braking point, and the next one will slide. A bit like riding down a country lane after frost - you just can’t ride flat-out in case you hit an icy patch that you didn’t see in time.