Maximum grade for NG RR lines?

As an archaeologist for the Forest Service, I’m mapping Narrow Gauge RR lines associated with an old logging/mining camp (1880s to the 1930s) in eastern Kentucky. The terrain is mountainous. The main rail lines on level terrain and the incline have been mapped easily enough, but with respect to grades I’m having difficulty distinguishing spur lines from mid 20th century logging roads.

My question is what is the maximum percent grade a narrow gauge engine could haul tonnage (i.e., how steep could they go)? I don’t know exactly the type of engine used, possibly a Shay. Horses and mules may have been as well.