The California Zephyr is a great way to get into the mountains from Union Station. Truly beautiful route, probably Amtrak's best IMO. More of a conventional train than a mountain train, but still a great route.

I remember the Royal Gorge is also a beautiful ride, though I haven't been for many years.

Apart from that, I hear great things about Durango - Silverton and plan to hit that someday. Durango has a lot of train stuff going on, might be worth considering.

Most/all of them are nice. Really close is the Colorado Railroad Museum, which had short (15 minute) rides behind a steam train. Georgetown Loop is a bit longer and very nice. Royal Gorge had nice dinner trains

If you are looking for longer rides, Amtrak to Glenwood, Durango/Silverton and Cumbres/Toltec are all stunning.

Royal gorge was sweet, we did the murder mystery car and won the game (not that hard but we're the only table to answer correctly)! Pretty good food too. Id probably try and do an open top car next time.

Georgetown loop was super short, like that's it?