Rail -> trail -> rail is rare, not because of the presence of the trail, but because the need for new rail to serve new financially productive traffic, on a line that shut down because there wasn't enough (or any), is rare. The trail does complicate matters, because the trail users are quite vocal about insisting that their rights be recognized and the trail be preserved in some way. Rail-With-Trail IOW. Each case, of course, is unique, and the dataset is very small, so there's lots of variety in how things turn out, but in the real world it almost certainly would not be a simple case of out with the trail, back with the rail.

As for reversion, it only goes by right to the adjacent landowners if they are successors to the original landowner who granted the easement. Yes, that is often the case especially in a rural area. It's not a given, though, and there have been many legal skirmishes when things got complicated. I have seen actual cases (P&SR for one) where it got complicated and took several years to resolve (well before Rails to Trails). Lawyers are sometimes worth the money.