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is that for one thing, so few were tested in any kind of standardised way; and also, that performance varied depending on how well the locomotive was being fired (in other words, crew skill). Tractive effort is a practical measurement in theory but, for a steam locomotive, a theoretical one in practice! It is calculated as the amount of effort the locomotive's cylinders can produce, given boiler pressure, cylinder size, and an approximated pressure drop-off. It's a static measurement. It measures what train a locomotive can start, but not how well it can pull at speed. Power is effort over time, or useful work. It was impossible with the technology of the time to estimate a locomotive's power output with numbers, so it had to be tested. Two ways: on a stationary locomotive test plant, which is what the PRR and some others did; or with a dynamometer car, that measures drawbar pull from the locomotive, distance travelled, speed etc. It's hard to compare the two results. The PRR Q2 was the most powerful locomotive tested on a test plant. The C&O's Allegheny was the most powerful tested by dynamometer car. It's probable the Allegheny was more powerful. Both locomotives developed their maximum power at fairly high speed, and were not nearly as powerful at drag freight speeds.
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