" 
 :::I cannot 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.   "