This peculiar locomotive hasn’t been run in decades, and (good) footage of it in action is really hard to find. The “single” in its name comes from the single driving axle, which is fitted with 8 foot driving wheels. That means that at 60mph, the engine is turning just over 200rpm.
Stirling was one of the last engineers dedicated to the single driving axle, instead of adding a second driven axle (and therefore doubling the wheels putting power to the rails), he reasoned it was more efficient to gain adhesion by simply making the single set of driven wheels as large as possible (larger circumference, larger contact patch). He had disdain for locomotives with multiple drive axles, because the connecting rods and counter-weighting needed to make them work were inefficient in his opinion (he still built them anyway, for low speed freight use).
This seems sort of ridiculous - but with the technology available at the time, it was a better bet to put huge forces on slow moving, heavy parts to move the train fast, rather than move smaller parts faster to get the same speed.
Anyway - look at how slow that wheel is turning. I would guess they’re probably not doing better than about 40mph, but you can see how slowly the mechanism goes for the distance covered (over 25 feet per revolution of the driving wheel).