Since time immorial the formula has been:- T.E (in lbs) = Cyl dia x cyl dia x stroke (all in inches) x boiler pressure (in lbs) x 85% divided by the driving wheel dia (in inches). However this assumes that the loco has square cylinders and gives an answer suggesting that the loco has a far greater tractive effort that it can actually achieve. Assuming 20" dia cylinders - 20" x 20" = 400 sq. ins area, against round cylinders pi x r squared = 10 x 10 x 3.142 = 314.2 sq. ins. Thus the generally accepted formula over-estimates the tractive effort by more than 27%. Is there a flaw in my thinking, or how have countless locomotive engineers got it so wrong for such a long time?