Unless the vegetation is a danger or obstructing signal sighting NR» cannot cut the veg down until the bird nesting season is over, also people complain when NR cuts "trees" down not to mention budgets are tight,
Just to add, they'll also (or at least used to be when I did this very same work some years ago) be removed if obstructing users sight lines at level crossings, especially user operated or footpath crossings. Certain species will be targeted as a priority to reduce the impact of the leaf fall season (e.g. sycamore) and of course any dangerous trees will/should be removed, hopefully before they drop any bits onto the railway!
I doubt anyone is particularly worried about a few leaves or light branches scraping along a train if none of the above criteria are met.