[quote user=“fwright”]
The equivalent grade for curved track came from John Allen. I have never found any evidence beyond it being Allen’s hypothesis for a formula for the extra drag created by curved track. It would have some basis on Allen’s experience in HO scale using trucks before the days of RP25 wheels and free-rolling wheel sets. A practical experiment (pure speculation on my part as to Allen’s conduct of any supporting experiments) would be to check how many cars a given locomotive can pull up a known straight grade as can compared to how many cars can be pulled around a known radius curve.
As a photographer, John Allen would likely have been familiar with 1/x type