timz
Steamlocomotive.com is even wronger than that -- it says two high-pressure cyl and one low-pressure. BLW 60000 was the only compound 4-10-2 in the US -- the UP and SP engines were all simple.
I no longer know quite what to believe, especially hearing that someone thinks that two HP cylinders of the stated dimensions could exhaust into one LP cylinder of stated dimensions in any practical sense of the word 'economy'.
A more practical* approach to compounding is the version used on the Triplexes as designed, where one high-pressure cylinder exhausted into two LPs (the high-pressure engine was in the 'middle', with one cylinder exhausting forward and then providing the boiler draft, and the other exhausting to the rear and thence through an escape-pipe minimizing the back pressure there).
As I recall and vastly simplifying without notes, the 'optimal' expansion ratio with reasonable limitation of heat loss being taken is something around 1:2.3 for a Smith compound, which is IIRC what Baldwin 60000 is supposed to be -- a practical modern design would probably use explicit reheat as well as intermediate modulated steam injection (a better version of N&W's 'booster valve') to equalize the effective thrust from the three pistons at any given steady-state operation. (In my opinion the greatest failing of traditional compounding schemes was not the "lack of expected power from the LP cylinders" but the performance and maintenance implications of lower effective thrust with a different risetime from the LP side per revolution working against the HP characteristics -- see Vauclain type 1 compounds for a fairly dramatic set of troubles so caused.)
The whole premise of Alco three-cylinder engines was to divide the drive and permit larger horsepower without increasing the mass of the reciprocating parts so needed. Much of what made this look attractive was the Gresley adaptation of Holcroft's conjugated valve gear for four-cylinder simples, which used a fairly simple linkage of pinned levers to derive the motion for a center cylinder's valve (with the valve rod facing forward instead of back) in a fashion easily adjusted for necessary angle, different size, or other characters of that cylinder. There are some characteristics of Gresley gear that would not 'give good service' over an extended service period without more careful maintenance than typical shop crews might provide. Note that it would be highly difficult, and even more difficult to adjust or maintain, a Gresley gear to provide the very, very necessary independent cutoff control for a LP cylinder vs. what the reverse provides for the outside HP cylinders, even if anyone were so stupid as to duplicate Webb's whopper of a misdesign.
In case anyone needs a good laugh, the Webb compound started by using two comparatively tiny cylinders for HP and one relatively colossal one (an account I've read terms this 'dustbin-sized' which is appropriate in the senses intended) in the center. To compound the felony (pun intended) the HP cylinders drive on one axle, the LP on another, and there is no mechanical connection or even conjugation between the two. Now, it would be difficult to find something even stupider to add to that, but Webb certainly managed; he did not see any need for a third set of expensive valve gear to drive that center cylinder, which after all would take only steam from the HP with its determinate cutoff settings, so the original version had only a slip eccentric that reversed 'itself' according to the direction the locomotive started moving under HP propulsion. This was an obvious improvement over trying to 'simple' an engine with so disparate a cylinder dimension, or just leaving the LP out of near-starting TE with the engine in reverse, and it worked great until you, say, backed the engine down on a train and then (as was almost incredibly likely, if you think about it even a fraction of a second) slipped the HP axle when starting -- this would result in the HP happily spinning 'ahead' and the LP obligately spinning up on the exhaust mass flow -- the opposite way as the slip eccentric wouldn't have been pulled to forward motion by the locomotive actually moving forward yet. One could not recover from this quite as elegantly as a problem with Winans valve gear, which required a starting bar to be inserted rather than some ad hoc reaching under the locomotive with something like a fire tool and pushing on things to set the LP in the right direction. A very good thing Mr. Webb knew Mr. Moon so well, I think. (At least a good thing for Mr. Webb, if not for the stockholders' interest or the folks who actually had to operate the railway...)
Apparently the later Teutonic 2-2-2-0s were somewhat better, and undeniably economical when running in steady state ... but as noted they did not last too long, once Webb was no longer dictator over the mechanical department, and no particular surprise.
To my knowledge (which is almost entirely derived from a quick reading of 'Three Barrels of Steam') there were no compound experiments tried with the Overland types on SP. In my opinion the UP Nines were an expansion on the 4-10-2 engines, and of course those were all obligate simples, so I find it hard to believe UP tried compounding either. Certainly the production engines were all simple, and perhaps the problem is that we have armchair folks seeing two different cylinder dimensions and unthinkingly concluding 'compound' from that.
*Perhaps 'less impractical' has more appropriate semantic meaning...
