I know that outgoing presidents usually leave a letter for the incoming president, and typically it offers advice like not spreading yourself too thin. Pick one goal, they write, and put 100% effort into it. If Biden follows this, I don't think the one issue he'll champion would be passenger railroads and Amtrak. There are just too many other higher priority problems.
However, I realize, I doubt Trump would probably never bother leaving a letter. Though, if he did, it would not be one of advice, but rather a rant about stealing the election.