Yes, that is a fantastic option, and why Minneapolis has such great bike path, I believe. But, you've got to have the tracks first, and I'm not sure many cities do. And even then, you can't choose where your paths go.

Or the train company owns it and expects to use it "someday" so they stonewall.

Are there any concepts where you can pave down the center and on the sides of tracks? To keep the track useful for later use, but to still allow cyclists to use them?

This strikes me as a terrible idea for two reasons.

1. Slim bike tires getting stuck in the grooves left for the train tracks.

2. Big trains with a ton of inertia running at ~50-60 mph using the same route as cyclists going at ~10-20 mph.

I'm not speaking of city tracks, where the rails are integrated with the pavement. I'm talking about tracks where they have ties in the center, and using concrete in the center between the rails to create a path.

Also, I'm not implying using it for high-speed trains and bike trails at the same time. I'm suggesting being able to go back to use of the paths for trains, and not tearing the tracks out.