Why are they chopping down the trees?



STEAM! Back in the days of old when steam ran, the heat and exhaust used to
keep any trees at bay and preventing them from encroaching too close to the
railway (with of course the odd fire thrown in)
However, as mentioned, since those days are long gone (electrification) the
trees themselves have pretty much grown as they wanted. They also cut them
down when they need to do bank stabilisation works.