Jeff,
I have both the Trails Along the Columbine that feature Marshall Pass, and the one or Monarch. Just doing some scripting recently and read that there was a lot of work done for turntables and strengthening things along the route for the K-36's. It did mention they were not used on the Marshall Pass line until 1926, which stacks up with what you said.
Our upcoming video has some shots of Marshall Pass snowshed with an excursion powered by 499 in 1947. They turned on the MP TT, but that is not seen(would be kinda dark for movies).
Okay, now back to editing D&S material for me.
Greg