The S-Line was an old railroad ROW, but you are exactly right in that the conditions that created the 'linear park' (as it is known in the biz') are not dependent on old railroad ROW's.
From a transit standpoint the S-Line Streetcar is really annoying, because it is one of the worst investments in actual public transit in the Salt Lake Valley while at the same time being the project that has translated into the most private investment. It sends the completely wrong message to decision makers about what sort of transit they should expand. That said, the S-Line trail and park are FANTASTIC, and I hope we see more projects them pop around the city (perhaps the
Folsom trail?)