The rule of thumb we used was that every degree of curvature was equal to 0.1% of grade. So a 10 degree curve was equal to a 1% grade. It depends on how old your railroad is. More modern railroads were compensated for curvature. On a compensated railroad, if a train is going up a 1% grade and enters a 5 degree curve, the grade will reduce to 0.5%. This way the locos will see no difference in the load on the drawbar. If the railroad is older and not compensated, the grade and curve resistance would add together resulting in an effective grade of 1.5%.
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