Not too many artifacts left- usually when they build a trail they remove things like whistle posts, tell tales, relay cases, and mileposts.
I've biked and hiked on several trails in this area where many artifacts were left in place. The Hudson Valley Rail Trail is a good example as is the Harlem Valley Rail Trail. Whistle posts, mileposts, and some signal equipment was left in place on both of these.
As for "in the hands of a developer..." what a developer gonna do with a 14-mile by 20-foot stretch of land that bisects private property?
Owning land that enables you to join properties would facilitate building something huge (strip mall, factory, warehouse, etc.). Granted, this will not happen in the middle of nowhere, but near towns (where stations were formerly located) it might happen. A classic example of this is in Elmsford on the old Put. Just north of Route 119, the ROW has been lost forever because it was sold to a developer. Now, a series of buildings and storage lots block the route.