Comment by lmm

If you want to hitch your steam train to the back of an Amtrak train and have it towed then you can follow the same rules as a private car.

If you want to actually drive your steam train then you'd need to negotiate with the track owner, which may be hard, particularly if they run on PTC (there's literally one ERTMS-compliant steam train in the world, for example). There's no public right of way on railway tracks for randoms, only for Amtrak (and even they have limits).