Hi, John,
It would appears that the real change in thinking about operating buses in the mountains occurred following a 1989 runaway that my district (Boulder Valley Schools) experienced in the South St. Vrain River canyon in 1989. After reaching speeds estimated to be about 70 m.p.h., the bus spun and then flipped over on its side on the river bank. One child was killed, another was ejected from the side emergency exit door and seriously maimed, and about about 25 other kids were roughed up and/or experienced a few broken bones. The structural improvements in school buses following the Monarch accident saved us from further carnage. Almost immediately afterwards, the state legislature mandated electric or transmission retarders on all school buses operating in the mountains, and now every new bus we buy has a retarder of some sort on it. Not only do they save lives, but they save a lot of wear and tear on brakes.
Speaking of those pre-WW2 Brill Rio Grande Trailways buses, I remember remember a trip about 25 years ago in a Continental Trailways bus (an Eagle) coming off the east side of La Veta Pass. It was dark; we were traveling at gawd-only-knows what speed. I was clutching the armrest in abject terror! Fortunately I lived to tell about it.
Mike