The limitation on sustained output on all our steam locos is the fireman. It doesn't matter how big the cylinders or how big the boiler. Even with two people firing, as often happens nowadays, there's a practical limit to how much you can shovel through the firehole door. At 2 ton/hr and an overall efficiency of say 7% that's going to come out at about 1600hp. Pushing that to 3 ton/hr will give 2400hp but that tenderful of coal will soon need replenishing And the not inconsiderable power required to drag the locomotive itself up any sort of gradient. For 140 ton loco on a 1 in 100 gradient at 75 mph, that's 626 hp according to my slide rule. Diesel or steam.