I can not do math The numbers are close but they should be exact Calculator vs slide rule I ve had fun looking at info sheets EMC EMD switchers to SW1200 say 57 or 100 ft rad SW 9 1101 specifically shows loco to car 57 or 100 ft radius and loco to loco same type 39 or 150 ft rad I went outside and looked at 100 ft radius That s tight But a SW 9 has trucks rotating on a 22 ft centerline Cars were smaller too Could you have done that in the stockyards or steel mills Our big interurban the C A E ran on the CTA s 90 ft radius and early on it did street running Smaller ones used streetcar lines They had radial couplers though No real loco could do either one of those But Whitcomb 44 ton 405 might 76 or 75 ft rad Anyway I think we agree that it probably should be 21 right That seems to be some sort of industry standard Have a good day