There was a really massive tree-cutting five or six years ago along the D branch. I'm pretty sure the number of trees removed went into the thousands. There were so many logging trucks on the road that Newton felt sort of like Oregon for a few weeks (except the tree diameters on the logging trucks were WAY smaller than what you see in Oregon).
This has very certainly produced a reduction in delays caused by down trees, though I do not have data. But in the years leading up to that tree-cutting extravaganza, it seemed like tree-fall driven delays were nearly a weekly event, and not just in winter. Every damn mildly windy day would bring down something big enough to drop the catenary wire. This is a much less common event now, limited to the kinds of days where it's easier to accept it (like yesterday / last night).
I have noticed that there were some more trees removed this fall. I don't know how extensive this was up and down the line. But some trees were removed from around Newton Highlands Station, where I spend enough time each day to notice such things next day (and a few of those removed were getting critical, so good riddance). And since then I've noticed a few other fresh-looking stumps out the window at other stops.