RJ Corman built a railroad line that in part used railbanked line.

[www.railroad.net]
[cs.trains.com]

There have also been cases where government agencies bought up abandoned (or about to be) railroads and held them, eventually in some cases building new rail or rail transit lines on them. The Folsom and Placerville SP branch is an example. Trails have been built along them, but so far except in El Dorado County trails have not completely replaced (removing the track) the rails.

Then there are the cases where abandoned lines were resurrected completely privately, like KCS' line in south Texas that bypasses a slow trackage rights segment on UP for its line to the Mexican border. There are also cases where railroads were smart and simply mothballed lines that were later reconstructed and put back into use; example: Stampede Pass.

That said, resurrecting a line that was truly "railbanked" - track torn out and a trail constructed - is certainly difficult and unlikely to happen. A railroad would have to have some really profitable business lined up to go through the hassle and expense of pulling a line out of railbanked status, especially if a trail has replaced the tracks.