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[quote user="caldreamer"] I have a pretty good tonnage rating system which I will use, since I do not fully underatand how to compute an RPA. Does anyone have any ideas on how to do this?[/quote] The tonnage rating system is good enough. SP got along with tonnage per locomotive type for many years. The prototype calculates the restance of a train depending on grade, curvature, bearing, rolling, air resistance. The Davis formula comes to mind. Union Pacific has defined 1 "equivalent power axle" (EPA) as 10,000 lbs tractive effort (TE). Knowing the resistance you can devide by 10,000 lbs TE and get the required number of EPAs. The UP System Special Instruction page 32 contains the EPA per locomotive type: <a href="https://www.up.com/ert/ssi.pdf">https://www.up.com/ert/ssi.pdf</a> In the 1980s SD45 and SD40 had almost the same tonnage rating across Donner Pass. Now you have different wheel slip software, different adhesion factors between DC and AC locomotives and that shows in the EPA per locomotive. For model railroading purposes a tonnage rating per locomotive for different grades is good enough, I think. Regards, Volker
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