Let me answer a more generalized question: What's the steepest possible grade on an adhesion railroad (excluding cog-wheel rack railways)?Typical maximum mainline grades in the U.S. are 2.2%, although a few are steeper. The steepest of which I'm aware are:
1) West side of Tennessee Pass, DRGW (3%, but now out of service
2) Raton and Glorieta passes on ATSF (both 3.25%, but ATSF years ago built an alternate main line further south, with much easier grades, that carries most traffic now)
4) A short stretch of Saluda Hill in North Carolina (NS, former Southern) has a grade of 4.77%. This is also now out of service.
Northern Pacific, Great Northern, and Southern Pacific maxed out at 2.2% over the Cascades and Sierras. The original ATSF line over Cajon Pass had a 3.25% grade, but a new line was constructed in the 1920s with a 2.2% maximum grade.
Grades are also generall compensated for curvature. There is a formula for reducing grade based on the degree of curvature (curvature increases drag).
While it's possible to climb a grade of 8% to 10% with an adhesion locomotive, it isn't done in regular service. Some logging railroads might have had grades that steep (I believe Cass Scenic does on the line to Bald Knob). You sure can't haul much of a train on such a grade.
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