My tonnage rating system works out pretty accurately for model railroad purposes. You take the tractiv effort of a locomotive, divide it by 160 for N scale to get the Tonnage rating on level track. Divide that number by the grade and you get a tonnage rating for that grade. For example. For an SD40-2 the Continuous tractive effort is 82,100 oiunds. Divide that 160 gives you 513.125 or 513 tons on flat andlevel tracl. Divide that by the grade, for exampe 2.93 percent and you get 175.127986348 opr 175 tons. I use grams of weight and use the actual weight of each car, so 175 grams will enable the engine to pull that much weight up the grade. A 500 gram train would require 4 SD40-2's to negotiaste the grade.