I was an electrician's apprentice in Barstow while going to college from 1964-1968 and we made up the consists for and did maintenance on the York Canyon train. Yes it was RSD15's, A LOT OF THEM, that made up the train. As I recall, I believe each train ran in the range of 10,000 tons, and the trick was to keep them under control going down Cajon Pass. What I remember (and I THINK this is accurate) is that in full dynamic braking and using the air when necessary, the train could be held to 3 MPH at it's slowest, that is to say, it could not stop on the grade down the pass.
There were locomotives in the front, middle, and at the end. it's a little rusty on how many, but I know when we made up the consists and tested the cabling in the diesel shops, the front end consist alone was too long for the big ramps. I think there were 3 units in the middle and maybe 5 at the end. I was thinking there were 8 at the front, but that those numbers with a grain of salt, I'm just going by the images still in my head and the fact that the consist was longer than the diesel shops.
The ALCO's were chosen because they were the heavy lifters of the time. Dirty and hard to maintain, but the management loved them because they were bulls out on the line apparently. From an electrician's point of view, they were a pain, always oil all over everything. They were called the "800" class, those and the "900" class EMD's were the big pullers. And those EMD were oily and a pain to work on also. 800's had generators, the 900's alternators, but both were still DC traction motors. There were problems at first with the radio controlled systems, and occasionally one of the DP's would go into reverse or dynamic braking. A big mess, cleaning up the grids on those when they melted down. Those were the days, I can recall coming out of the shops (apprentices workd 1st shift M-F) one August day and the temperature sign on the local bank read 122! You had to be young to work in the shops during the day, all the old heads took M-F night jobs in the summer.
Don't know about what happened in the 90's, long gone by then.
Hope this helps a bit though.