If my memory is OK this morning, the Uintah Ry. had several 50-2 class 3' gauge Shays and they were regularly able to negotiate the 66 degree curve at Moro Castle.
The 8-coupled engines at the upper end had to be put on flats and hauled over the line when really heavy work was required. The pair of 0-6-2T and the two 2-6-6-2Ts got over Baxter Pass OK though.
As an aside, that is a fascinating hike for anybody so inclined. I did part of it with a couple of friends around 2003. Well worth the effort and time.