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[quote user="erikem"]  This relates to La Massena's thesis in his article "The Big Engines". The UP could have made up their trains to make use of the Big Boy's peak tractive effort of a bit over 120,000 lb, but the train would have been slugging up the hill at less than 10 MPH. By dropping the tonnage by 5%, the UP got 50% more speed and were less likely to stall on the hill. I was also getting a figure of a bit over 4% for thermal efficiency from the Big Boys from the data in Kratville's book. I wonder what tractive effort and speed combination would give the best thermal efficiency? A WAG on my part would be say 40,000 lb at say 30 MPH, where the throttle would be wide open, the reverser hooked up as tight as can be to get maximum expansion.  The locomotive would be running at less than maximum power which "should" help improve both combustion efficiency and heat transfer efficiency. - Erik [/quote] Thank you Erik, as you mentioned it, does somebody know what peak tractive effort BB could excert? With a 4000tons train it was 115.000lbs, final ratings were 4450tons, 10percent more, so 126.500lbs? I could never figure it out, my guess that would lug BB down to 5-10mph. Kratville stated, "later, they performed so well... in the final years of steam they were given 4450 tons". The 4450 tons rating occured 1948, btw. However the book leaves us railfans pretty alone about that. Cheers lars
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