When the second portion of the RGS (Rico to Durango) was certified for Mikado operation on 1/1/28 it was for locomotives up to the size of the K-28. I've never seen nor heard evidence of one venturing west of Durango however.
Don Rogers recalled riding on the "Edna" in 1939 while some of the RGS personnel were discussing URY 40. URY wanted more than the RGS was willing to pay. The RGS was short of their own motive power, particularly at livestock time, and had looked at a few different locomotives for possible acquisition during the 30s.They even dispatched Randow to survey some candidates.