It looks similar to the “Unit Steam Engine” that I use in my Heilmann (except there are 4 of them there)… I have had no real problem with the simple “ossy” and I actually prefer them to the more “correct” type.
Those are very interesting. I didn’t see any reference to size. Was it not there or am I just blind? Keep in mind also, that those engines will run on compressed air or nitrogen. A paintball gun tank in a boxcar would make a good air tank.