Many of the old rail lines are being converted to bike paths in New England.  But I guess it wouldn't be too hard to convert them back to rail lines if need be.
Unfortunately, trail to rail never happens.  Experience has shown that once the hikers & bikers get a corridor (even with a proviso that it is "reserved" for future urban rail use) that they are unwilling to share (most US RR ROWs are 100 feet wide, plenty of EU examples of narrower shared rail & hike/bike ROW) with a new rail line.

Thus, the advice among rail advocates is let weeds grow and not allow development of hike & bike trails.  Else the future  rail line will have the added burden of dealing with activist hikers & bikers which may kill the entire project.

Poor state of affairs, but that is the US reality.

Best Hopes,

Alan

Unfortunately rail to trail never happens.

That's because we've had trucks running and we haven't needed to, yet.  Cynus is right that those lines are being preserved by being bike trails for when we need to convert them back to rail lines.  The reality is that the lines that haven't been preserved have gone back to the farmers/private landowners and will be very difficult to ever convert back to rail lines.  I'm an avid "Rails to Trails" member and railroad supporter as well.

The advice I have heard from others is, if the transit authority or other gov't organization can buy the old RR ROW,is to NOT convert it into a hike/bike trail.  There are several cases where long range plans for light rail lines on old RR ROW could not be converted into actual plans because of hiker/biker resistance.  For all intents and purposes, the ROW was lost for transportation due to recreational demand.

Urban Rail lines were not built due to the unwillingness of hikers & bikers to shars the ROW (there was and is room for both).

In New Orleans, I try hard to keep good relations with the bike groups.  For my Desire Streetcar plan, we replaced 4 traffic lanes with 2 traffic lanes, 2 bike lanes and 2 streetcar tracks.

Best Hopes,

Alan