Around 1948, the Grande promoted their first batch of engineers in many, many years (like the 1920's). There was quite a crop of firemen hired in the mid-late 1920's that worked as firemen for over 20 years before getting promoted. Most of the old heads they fired for were fossils that had been sitting on the right hand seatbox since the early 1900's having been promoted within 5-10 years of hiring on. Because that many of the late 40's promotes had little experience sitting on the right side and running. With that in mind, once they got on the right side, they didn't let their younger firemen do any running either. Jim Pearce, who hired on in 1948, told me when he got promoted in 1955, he had had little experience in running.
On the other side of the world, Alamosa was a bit easier on it's firemen. They too had a long list of old heads that would glue their butts to the seatbox and never move. But the guys who got promoted in the late 40's were a more "progressive" group that let their firemen run. Eldon Morgan told me he got a lot of throttle time running down hill from Cumbres and on the SG engines that were stoker fired (OK the engineers weren't going to work THAT hard...). He said when he got promoted, he's let his firemen get some right hand seatbox time, as he actually liked to fire the engines ("it kept me young..").
Granted, the topography had a bunch to do with it. The east end is all flat, uphill and downhill for long stretches. So, an engineer could put his fireman on the seat at Cumbres and not do a whole heck of a lot all the way to Antonito and beyond. Whereas the west end was a rollercoaster. Up hill for a ways, then down, then back up again, a lot more work going both ways. The longest stretch of downhill was from Willow Creek all the way to Arboles.