Once I read in Railroad Magazine how to estimate steam horsepower. The story said the square feet of the boiler heating surface area,not including the superheater,is roughly equal to the locomotives horsepower. This was in a 1940’s issue of the magazine. Joe
It’s a pretty coarse estimate, but there is some basis in fact behind it. The amount of steam generated depends on boiler heating area and the amount of power available depends on how much steam is available. One of the areas where it falls down is that heating area in the firebox is much more effective than heating area in the flues/tubes.
I thought I read somewhere that the 2-6-6-6s had 7,498 drawbar horsepower. Which would put them up there at #1 for steam horsepower. Odd that GE and EMD put out 6,000 HP engines but the railroads decided that mid 4,000s was actually better.