Lehigh Gorge Scenic Railroad / Reading and Northern RR in PA..//. Cass Scenic RR in WV - st Cass, you can camp in the mountains in a caboose. Then Ely, Nevada for the Nevada Northern. And lots more around. Cumbres and Toltec in southern Colorado, 64 mile trip.
There is one in Cumberland, MD that runs in fall and maybe spring thru the mountains to Frostburg. They have just finished refurb, or about to, what will be the largest running locomotive in the country they say. Unfortunately, they have just determined all the ties must be replaced on the tracks at several million they don't have, so do not know when it may run again.