I was in Vietnam when the equipment was moved in 1970. I made my first trip in April of 71. A lot of the 3rd rail between Alamsoa and Antonito had been pulled. The dual gauge was still in place in the Alamosa yards. A large number of stock cars were stored on a segment of the former Valley Line just across the highway. The D&RGW was still moving ng cars around with dual gauge idlers in April. I saw a GP9 move some stock cars around the yard. The brake beams had already been removed from some of them. There were stacks of car parts all around the yards and I got a photo of an 18wheel flat bed truck loaded down with 8 6500 flats. The trucks were loaded on another flat bed. I was told them were going to South America.
Doesn't answer your question, but best I can do with what I saw. Perhaps someone else knows when the last dual gauge train ran to Antonito?