I'm not sure which ones offer nighttime sightseeing trips, but every single one is in the southwest part of Colroado, and much farther south than Fort Collins. Where are you going in New Mexico? Adding a train trip will likely add at least a full day if you want to do a roundtrip, unless you are doing a shorter railroad like the Georgetown Loop.
Roughly in order from north to south:
Georgetown & Silver Plume (Georgetown Loop RR - only about 4 miles long but they do have a silver mine tour as well)
Leadville (Leadville Colorado & Southern RR)
Cripple Creek & Victor (Cripple Creek & Victor NGRR - there's a nice gold mine tour in the area)
Canon City (Royal Gorge Route RR)
Alamosa & La Veta (Rio Grande Scenic RR) - probably the closest to I-25 (for La Veta, anyway)
Durango & Silverton (Durango & Silverton NGRR) - southwest corner of CO
Antonito (Cumbres & Toltec Scenic RR) - one end is in CO and the other is in NM, roughly halfway between I-25 and Durango. Probably the longest, with D&S being a close second.
I've been on the Georgetown Loop, Royal Gorge, and Durango & Silverton. Of those three, the D&S was my favorite. I'm hoping to ride the C&T this summer.
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