Rich,
Was the engine on sand anywhere on the climb? And do you know the percent grade? At low speed, near stall, the engine was not making much drawbar HP, but was without question at maximum boiler HP, all of it going into generation of tractive effort (i.e. drawbar pull). And do you have an estimate of trailing tonnage?
My guess is that, in steam days, the engine's tonnage rating on that grade would be such that the train's minimum speed would be in the ten or 15 mph range.
