About 50 years ago, I asked Bob Richardson of the CRRM about this operation. He saw this operation a lot; he used to live in Alamosa. The Chili Line baggage/RPO car was in front of the San Juan consist, and the Chili Line coach was at the rear of the San Juan consist. The San Juan loco would pull the combined consist from Alamosa to Antonito, with the Chili Line loco waiting on the Chili Line, just south of the junction switch.
The train would stop with the cars north of the junction switch, the San Juan loco would uncouple and move forward onto the Chama line light. Then, the Chili Line loco would back on to the consist, couple to the Chili Line baggage/RPO car. The baggage/RPO car would be uncoupled from the San Juan consist, and the Chili Line loco would pull forward onto the Chili Line with just the baggage/RPO car.
Then the San Juan loco would back up, and couple to the San Juan consist. The Chili Line coach would be uncoupled from the San Juan consist, and the San Juan would pull forward to do its station work. Then, the Chili Line loco and baggage/RPO car would back up to pick up its coach. Then, the Chili line train would pull forward to do its station work. My understanding from a description by Forrest Crossen is that they would ring the loco bells a lot while during this switching.
Robert