The best 4-8-4s would give around 4000HP in normal hard work, high 4000s for peak efforts and nearly 6000 if showing off. Other latter day 4-6-4s should show similar behavior. But these should be viewed as short term stunts rather than daily capability. This needed about 5000HP, also needed for the reported 5 miles at 120 mph. On one recorded run, an F7 ran briefly up to 113 mph. Analysis of Hiawatha running shows the F7s would develop about 4000 IHP when accelerating, then back to ‘comfort zone’ 3000-3500HP once up to 100mph. The NYC said that ‘above average’ service operation for a Niagara was 4000HP. These figures are far from what you can expect in normal service (coal consumption, boiler maintenance costs etc). It is well recorded that both T1s and Niagaras could achieve 6500+ cylinder HP. It would take forever to accelerate to 100mph at these powers, so you need to add 1000HP to these figures and indeed the AAR concluded in 1938 that you needed 6500HP to run 1000 ton trains at 100mph. ![]() So to maintain 100mph, a Hiawatha of 825 tons total, needs about 3200 cylinder HP on the level a 1470 total tons T1 hauled Trailblazer about 5400. For US types, the train as a whole needs 3.5-4 HP/ton to maintain 100 mph on the level. To estimate HP/ton needed, we need to know the ball park for locomotive and coach resistances. But fewer, heavier, lower HP/ton trains made more sense. An NYC Hudson could run a nine car Hiawatha (similar weight to the loco) at 100mph most of the way from Toledo to Elkhart. Rather, high speed is really about two things, HP/ton and help from gravity. An NYC Hudson was slipped up to 164mph, so no problems with the robustness of US motions at speed. So what made the MILW types the fastest? Nothing to do with design really, I say, They basically just represent 1930s best US practice though the 84” drivers will reduce piston speeds and maintenance costs a bit. There is nothing else approaching this anywhere in the world with steam. Speed often flat lined at 100mph south of Milwaukee from around Lake to Forest Glen. You can’t average 81 mph from Sparta to Portage, uphill start and slow through Tunnel City without then reaching 100mph quickly and staying there whenever possible. My definition would be ‘what regularly ran fastest in daily service?’ This, as far as I can tell is a two horse race between the MILW types, scheduled to run at 100mph over long distances four times a day. I will go on to argue this is near the limit of the possible. The usual one is ‘what holds the speed record?’ and here as far as hard data goes, the fastest are LNER Mallard and the German O5 at ca. ![]() Further, the locos I believe have by far the best claim to “fastest ever’- the MILW F7s and As - got little mention! In part it’s a definition thing. This had a lot of fun anecdotes, though little hard data. ![]() I missed the earlier thread on ‘fastest steam locomotive’.
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