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Ironsideroger:

Not sure how well naval propulsion matches to RR. Important to distinguish tractive force from HP on the railroad - not sure if there is an equivalent distinction in ships (never thought about it).

Steam locomotives were specifically designed to have (a) a tractive force that could be applied without excessive slippage, and (b) a peak horsepower speed that matched the task at hand. The I1s was specifically designed for peak HP when dragging a train up a grade. Note that the HP required to pull a train up a grade of a fixed percent INCREASES as the train's speed increases. So the I1s was effectively "geared down" by using smaller drivers, allowing more HP at low speeds so that the most tonnage could be pulled up hill with each locomotive - and that means slow speeds.

Conversely, for a passenger engine, more HP is needed to accelerate the train, and to overcome wind and rolling resistance at high speeds, which can be quite significant (I think I once read that the T1 applied over 1000 of its horsepower at 100 mph just to move the air out of the way (form drag)).

So the PRR, wanting to operate on the west end at high speeds, designed the T1 with the peak HP point at a much higher speed than diesel's of the era (much, MUCH higher than an I1s, and higher than a K4s), and raised it a little further when they reduced the cylinder bore of the T1a's. The higher peak HP speed provided faster accelerations at high speeds (especially to get back up to speed after going through curves that requried speed reductions), plus a higher cruise speed (admittedly the engine may have had surplus power at high speeds on the flats, but that power could be used to maintain high speed going up modest grades).

While the T1a change reduced the TE, and the hp available at low speeds to accelerate the train, it increased high speed horsepower and therefore high speed acceleration, thereby reducing total time to get to speed. Since the T1 was considered to be slippery, the reduction in TE may not have been noticed much.

The steam engine probably corresponds quite well with a car that only has a clutch - no transmission. You can pick an axle gear ratio to pull tree stumps, and redline at 5 mph (a shay), or a gear that can pull a thousand tons up a steep hill at 10 mph (I1s), or a gear that applies the engine's peak HP at 100 mph. For this gear you'll need to ride the clutch for a while lest the engine stall at low speeds (in the steam engine, the throttle is cut back so the wheels don't slip). You'll also never pull a stump with it.

The diesel's, with the combination of motor, electric generator, and traction motor, could apply max HP over a wider range of speed - low speed HP would be limited by traction motor currents and motor heating (from the currents - but at low speed HP isn't really that important), and at high speeds you would be limited by the prime mover. Closer to an automatic, variable speed transmission.

Recent articles (and rediscovered documents) seem to be renovating the T1 design concept (except for the poppett valves). And clearly some reports indicate they ran well when well-handled by the crew.

Too bad they were all scrapped! What a great engine to restore. I'm getting tired of all of these steam excursion videos of other road's northerns - the T1 would have whipped them all in territory where it could be turned lose.

Regards,
Dave Evans