Hi John,
Looks like you were there at just the right time. My father was there a few years earlier on a Karl Helft tour and if I am remembering correctly he said that apart from the regular passenger, there wasn't much going on. They even approached the railroad to see if they could run a charter mid week but were told "impossible, we are too busy." Dad said during the day they spent at El Maiten, someone would get in an engine and run it the length of the yard and back once an hour or so but that was it. He said it looked like a classic make work operation. Apparently the dam construction really did keep them busy during your visit.
When I was there in 1991 they had gone from a twice weekly mixto to a once a week mixto. I stopped in the dispatcher's office in El Maiten on a Wednesday and asked them for the weekly schedule. He wrote it out in a beautiful copper plate hand listing the weekly passenger trains and a freight on Wednesday. Overjoyed, I headed out to the car and then realized that "World Steam" had said there had been no freights for two years. I went back into the office and handed him the schedule and asked him to tell me if anything was working today. He wrote "annulled" on the freight and said there was a light engine move from Cerro Mesa to Mamuel Choique to pick up two failed engines that would be leaving 11:00 "mas o meno." We jumped in the car and took off for Cerro Mesa, arriving just in time to find one of the Baldwin's with the blower on ready to go. We followed them out and back, although they were only able to bring back one loco. The other apparently had some running gear issues that prevented it from being moved.
I learned to always make sure you ask the right question!
Michael Allen