Third railing the Antonito to Alamosa tangent would be only one element of a very costly infrastructure. Once the rails were in place you would need additional locomotives, cars and train crews. Probably need to spend some serious bucks to build and operate locomotive servicing facilites in Alamosa also. Also an additional 2 hour journey time added to the Chama/Antonito trip would not be marketable.
That said, there might be another alternative.
If the Alamosa/Antonito line could be inexpensively upgraded to perhaps a 50 mph passenger track speed, then there might be the option to employ a shuttle service perhaps using one of the new prototype rail busses manufactured by Colorado Rail Car.
Regardless, the Alamosa/Antonito line would hardly qualify as a "Scenic" railroad.
The capital costs for any type of Antonito/Alamosa rail service would be high and the area would be better served by continued tourist destination development and promotion of Antonito.