Easy, this is likely in the evening. The photographer rode the San Juan from Alamosa to Durango. I suspect this is after he arrived and their work is done. 315 was indeed the switcher and the next morning he has some film clips of it switching. Then we see K-27 453 moving around, and it powers the Silverton Mixed which he rode. 40's material is seldom seen in films, which makes this all special in my book. There were a ton more sill images from the 40's than movie films.
Greg