"Railbanking" with a trail doesn't abandon it as a railroad. The trail use remains subordinate to the rail use, if anybody ever wants to reinstate the trains. In essence, the railroad still holds the right to use it, so it doesn't officially revert to the underlying land owners, assuming that, as was common in the old days, the railroad has an easement rather than fee title.

As a practical matter, once the rails are gone and the trail is there, it's very difficult to reinstate the rails. It's happened, but only rarely, and the railroad usually is forced to do something to keep the trail in the r/w. RJ Corman did it in one case. It would be interesting to know if a test case has ever been litigated (with references) about railbanking, and whether at some point in time it's considered effectively abandoned as a rail line so reversion can be settled one way or another.

Reversion, of course, would be to the original grantor of the easement, or their successor. It's not, necessarily, to the current adjacent landowner(s). In many places, that could make for a complicated legal situation. Having some agency with public funding (and therefore deep pockets for liability) maintain it as a trail might actually be a better for all concerned than just leaving the mess to sit there. Abandonment doesn't always mean that the old rails, ties, crashed trains, and bridges will be removed and the site restored to some usable state.