Acceptable grades

[Sumpter250]

Dividing 10" by 180"(15') gives you an 8.33% grade. So that is a very steep grade. But the narrow gauge Uinta Ry had 7.5% grades w/ 85 degree curves & ran 2-6-6-2s thru that! So it depends on your prototype. Santa Fe's Cajon pass has a 3% grade on one track but mostly runs trains down grade.

OK The grade I quoted as 6% is probably the same one ( 7.5% ), and I did forget about the tight curves !!
The angular size ( degree ) of a curve is the angle formed by two lines drawn from the center of the curve, to the points where where a 100' chord, intersects the curve center-line.
An HO scale 18" radius curve, is a scale 45 degree curve.
An 85 Degree curve would have an approximate radius of 75' !!. In HO scale that would be 10-3/8"

My N scale 2-8-0 can operate down to about a 9" radius comfortably, My HOn3 Brass 2-8-2, "complains" about an 18" radius curve, My HOn3 2-6-6-2 T's ( SVRY 251, and 250 ) might handle a 15" radius curve, but they would also be "complaining" about it. In HO scale 15" rad. = a 66 Degree curve.