It sounds as if the goal is to see fall colors from a train.
I would suggest a railroad that is actually in the mountains, such as railroads from Leadville, Cripple Creek, Alamosa / Antonito, Chama, Durango all of which have nice drives through the mountains to get there and the fall colors are visible from the road. It makes the whole trip nice. It's only 6 hours from Denver to Durango so most of these railroads are within a day's drive.
Or they can come to North Carolina and ride the Great Smokey Mountain Railroad.
With all the dry weather we're having the colors are starting to change. And they probably don't need winter jackets (heard it was really cool on Sunday. It was only 98 at my house near Asheboro, N.C.
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If they want to see trains and are around Denver, the CRRM is handy being closeby but not so scenic as these other railroads. You can spend HOURS there easily.