This happens a lot. But the new road/rail often shares the line of the old on the easy sections, so you may not get a continuous bike trail out of the old route

This happens a lot. But the new road/rail often shares the line of the old on the easy sections, so you may not get a continuous bike trail out of the old route. (Plus, as you say, old bridges/tunnels/steep slopes may be unsafe, or simply damaged & not repaired.)