I found this on the Uintah web site
"Geared type Shay locomotives were used for the steep grades and sharp curves over Baxter Pass as the Rod type engines could not function under such circumstances. The Rod type were used on either side of the pass where the curves weren't so sharp and the grades not near so steep."
I understood they took a NG Mike up to the top of the RR on a flat and only used it at the top, not on the sharp curves and steep grade. The articulateds were purchased as replacements for the geared locos to use on the steep grade and sharp curves.