Thanks for your post, Bill!
Your second paragraph gives a good example of how steam, even though flowing through a small line, can do big things when it accumulates behind a piston.
I just saw another interesting example in an old issue of Railway Mechanical Engineer, about the Pennsy's Class I-1s decapods. At the time they were designed, they were notable in having a very short cutoff in full gear—only 50 percent. The purpose was to make more use of the expansive power of steam in drag freight service than had been typical.
But with a full-gear cutoff that short, a locomotive stopped at various positions of the cranks wouldn't have been able to start. To get around the problem, the designers included a pair of tiny auxiliary steam ports at each end of the valve chamber on each side of the locomotive—four tiny pairs in all. The combined cross-sectional area of one pair was only 3/8 of one square inch. But when the steam passed by one pair accumulated against a 30-1/2" diameter piston, the force was enough to start a heavy freight train.
There was no provision to close those auxiliary ports. I think the reason was that once the locomotive got the train rolling at even a very slow speed, the action of the auxiliary ports became negligible. They couldn't pass steam fast enough to be a factor once the engine got rolling. I presume the concept was successful, because one website I consulted states that the Pennsy acquired 598 of those decapods. I think that same site mentioned that the original design was modified, but I don't know if any changes involved the auxiliary ports.
