[bombcar]

Railroads are usually loathe to give up right-of-way even if it has been abandoned, but it can be done with enough support and money.

Many rail trails are still owned by the railroad with an option to make them rail lines again if they want.

[ghaff]

Yes, where I live in the greater Boston area, you've seen a few commuter rail extensions that I assume made use of existing railroad right of ways that were abandoned (ADDED: As in no longer used for rail service) at some point once the rail service became commuter rail service