We had trains as theme one year here in Colorado. There are a variety of trains that one can do, and can actually be done relatively easily, though not cheaply. The trains we did were:
The Georgetown Loop in Georgetown CO
The Victor and Cripple Creek RR, between those 2 towns
The Silverton & Durango RR, between those 2 towns.
The Cumbres and Toltec RR, from Antonito CO to Chama NM.
The Pikes Peak Cog Railway (late great train ride-will be missed)
For Silverton & Durango, one can take the train both directions, or take the train up to Silverton and a bus back to Durango.
For Cumbres and Toltec, there are a variety of options for this trip, starting from either point, and involving a bus ride in the opposite/return direction. Or one can choose to go half way up, and return to your starting point, from either direction. IMHO, the best way to go is from Antonito to Chama, as the geographical change is more gradual that direction versus starting in Chama.
One we did not do, but should be considered as well is the Royal Gorge Route Railroad, near Canon City CO. There was also the Royal Gorge Scenic Train, a miniature railroad that one could take (think kiddie train, but for grownups as well), that would take you out to an isolated viewpoint and then loop back. Unfortunately, they stopped operation in 2012.
