On the D&RG most switching was done with old 2-8-0s that were often outfitted with step pilots for the switchman to ride on between switches and uncoupling cars. C-16 number 271 did the chores for some years in the Durango, Colorado yards. It sported a tool box on the cab roof. A number of other various C class consolidations also made Durango home assigned to switching as well as other locations.
Early on the D&RG had I believe about three 0-6-0T locos for switching. One of them went to the RGS as well. I don’t think they lasted long.
A major reason there were so few specifically built switchers on the narrow gauge lines was the fact that most railroads standard gauged their operations or were built that way resulting in an ever diminishing number of narrow gauge railroads. As a result they became regionally isolated and used narrow gauge locomotives were abundant. Too, being small engines mostly to begin with most weren’t worth standard gauging for resale. There were few cases of interchange of cars either between the ng lines because of the isolated nature of them although there were a very few instances.
If you simply want a small lokie then there are a few to pick from. Besides the early 0-6-0T steamers the D&RG had a small 4-wheel diesel or gas later at Durango as did the SPs Keeler branch and the West Side Lumber Co. just to name a few. In addition many a small mining or gypo logging line in 42" 36", 30" and 24" gauges particularly had tiny 0-4-0Ts from Porter and Baldwin and others.
42" gained a temporary popularity in the US on small logging lines because of the large number of used steam dummies that became available when major street lines electrified in New York and elsewhere. Many traction lines were 42" gauge. These little lokies were cheap and simple and were commonly operated until they were used up and then left abandoned when an area was logged out. It was usually cheaper just to buy another one for the next operation than to pay to haul the old one out of the woods and overhaul it.