As a member of the UP Historical Society, I received the latest Streamliner publication they put out. Vol. 34 No. 1. Inside is a great article by James Ehernberger and Marc Entze, pics and mechanical diagrams of the steam powered rotaries. Also a narrative of a 1956 trip to plow the line from Aston Id, to West Yellowstone. If anyone has traveled there in winter to snowmobile, you can appreciate the amount of snow that can build up south of Island Park. Nowdays, there is a nice museum in the old UP Depot at West Yellowstone. Finally a trip to plow on the Colorado plains where drifts are intermittent but deep and crusted, used a D-9 cat to break up the crust ahead of the plow in 1977.
I'm also a member of the UPHS. Excellent articles on the steam rotaries. I've learned much about these machines, including those last one built. Athearn's HO scale model is based on those rotaries.
I read the article in the Streamliner last night. Up to that point, I didn't understand how they were worked. It was interesting to me that the last two, built by Lima, used a 3 cylinder engine similar to a Shay to power the rotary wheel. I have a Video Rails Hank Griffiths collection video that shows a UP steam-powered rotary in action that I haven't watched in a few years. Unfortunately, the video is on VHS and my VCR stopped working a while back.