I have to imagine it's a similar scenario to pilots taking over your vessel in certain ports and passages. The job of the pilot (or the Amtrak hired crew) is that they are aligned with the port's goals and understand their scheduling and rules. I can't imagine they would want a private car moving of it's own accord operated by the owner, so you might still have to hire the Amtrak crew, and they might not be willing to operate your personal car since crew have to be trained on the locomotives. I am speculating though, I have no idea.