I have been doing some reading in the Withun book about steam loco development and came across an interesting engineering datum developed in the 'teens by Cole. Apparently, firebox evaporating area is almost 3 (2.7, to be precise) times as effective as flue heating area. Flue area is good for 10 lbs of steam per square foot per hour; firebox area is good for 27 lbs per square foot over the same period. I have not yet discovered whether a figure exists for combustion chamber evaporative area, which one would imagine is somewhere between those two. Firebox evaporative area is not a linear function of grate area: a large grate suggests a large evaporative area, but the height of the sides of the box is a variable not dependent upon grate area. Certainly, a large grate are DOES imply a large crownsheet area. In other words, total evaporative surface is not enough: it has to be the right ratio of kinds of evaporative surface. Anyway, this pair of constants in the HP equation helps to clarify why and in what proportion) the ratio of firebox to flue evaporative area is crucial to the power of the locomotive. Whether or not the 2001s had a problem in this respect is not yet clear to me, but it was well understood by the engineers (building types, not running types) of the time why many drag freight era engines were not performing adequately. Superheat helped to address the problem; super power (i.e. enlarged fireboxes with much greater evaporative surface are) helped more. Enter Lima, Stage left.