Ross,
I don't know exactly what the rail in pounds was on the Crested Butte branch and all my books are packed up since I have moved recently, but I can tell you that up until the early 50's the Crested Butte branch was mainstay of traffic for the ng and I bet the Crested Butte branch had 85# rail in places and the K-37's where the main power used throughout the 40's. I'd assume track upgrades probably took place in the late 20's and early 30's as the K-36's and 37's came on line. Sorry I couldn't be more specific.
William
aka drgwk37