most powerful locomotive ever built
Below that and you have wheel slip. Anyway, he lists the N&W A with 6300 HP, the Union Pacific Big Boy with 6300 and the C&O Allegeny with 7498. I had thought that the Allengeny with about 7500 HP was the most powerful reciprocating steam locomotive. To a great extent the horsepower depended on the boiler capacity and the ability of the locomotive to run at a relatively high speed (power = traction effort X speed).