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[quote user="NWP SWP"]'ve run four SD40-2s in DPU (two leading, two more 20 cars in) on a 40 car train, it had trouble getting up the 3% grade of the helix, had I put say a pair of GP20s on the lead as helpers it probably would've handled it,[/quote] Question: Usually there is the nominal grade of the helix plus the resistance caused by the curve. John Allen found an equation to calculate the fictional grade cause by the curves. g(curve) = 32/r;  r in inch, g(curve) in % For a 3% helix with 24'' radius. Grade = 3 + 32/24 = 4.33% On model railroads helpers or DPUs work best when they are really needed to get a train up the hill. Otherwise the inner train dynamics might lead to problems. These will not occur with head end helpers. Regards, Volker P.S.: For those interested in train physics like tractive effort and train resistance here is a slide show: <a href="http://slideplayer.com/slide/4696076/">http://slideplayer.com/slide/4696076/</a> All is based on the Davis equation which considered parts that are speed independent, dependend on speed, and dependend on speed². The coefficients were empirical developed. Over time the equation was adjusted to developments in the railroad industry, e.g. roller bearings.
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