Sometime back around 1968-70, Trains featured an article, possibly two, about what might have been if a “stumble” or two had allowed one more cycle in the refinement of steam power. The ideas revolved around things like cross-connected “balanced” drives involving four cylinders – two at each end of one set of drivers, and the end products depicted were a 4-10-4 built around the idea listed above and a 4-8-6 that represented a “souped up” Lima “mass production” Berkshire. Another feature I recall from about the same time argued that very little refinement in steam technology emerged after 1928; it didn’t actually "rank’ various classes of steam power so much as argue that the later products of the steam locomotive builder’s art sometimes didn’t pan out because they didn’t fit into the service for which they were eventually assigned – WM’s 4-8-4’s and P&LE’s Berkshires (Alco’s last steam order) as prominent examples. Conversely, the Decapods which were Lehigh and New England’s last seam power drew high praise.
I think you have got this a little sideways: the ‘balanced’ Withuhn drive requires an even number of drivers to work correctly (you need a Deem-style geared conjugation to do a 4-4-6-4 or similar) so it would be eight-coupled or 12-coupled (the latter almost certainly having so long a resulting effective rigid wheelbase as to make a modern Challenger a far better approach).
The souped-up Berkshire would be a 2-8-6; there was no reason on any fast-freight engine to use a four-wheel lead truck. The 4-8-6 was actually pitched by Lima (see Hirsimaki’s book) but as a better 4-8-4 dual-service engine.
It’s easy with 20/20 hindsight to make claims about ‘practical steam technology improvement ending in 1928’ – but in actual fact, quite a few of the real significant changes came well after that date, and no few cockamamie misadventures predated that. (Amusingly enough, one of the great dead ends, the Central Machinery Support 2-12-6, was pitched to the International Railway Fuel Association in 1928…)
Look at Alco and its assumption that three-cylinder power was THE shape of the future as seen from the mid-Twenties, or anyone’s experimentation with the Schmidt system or other approaches to very high working pressures stemming from power-station technology improvements after WWI. Look at the massive trouble inv
Duplex/Deem drive are maybe in the gimcrackery category. Yes, badly designed, badly operated, or badly maintained steam locomotives could bend rails with their dynamic augment “hammer blow” and wreck large sections of track. But the sense I get (from the cough, H. F. Brown paper, cough) is that you could balance two-cylinder locomotives to not be hard on track. The most recent thinking is to underbalance them, that is balance against the rotating mass but not the reciprocating mass, and to have yet-another-Franklin-gadget in the form of an articulated connection with the tender take up the back-and-forth surging that results.
But what else was a gimcrack gadget? The feedwater heater? The superheater? The Valve Pilot? Two of those gadgets made substantial contributions to water economy and in turn fuel economy, and by increasing those economies, you also increase the HP limit of the locomotive. The Valve Pilot allowed doing more with less locomotive and also gave important diagnostics.
And modern drivers, and cast engine beds, and lightweight rods and roller bearings
The conjugated drives are only significant if you are running high cyclic rpm and have a need either for ‘diesel-grade’ levels of track maintenance or high-speed alignment.
You are correct that for most American needs a two-cylinder engine with lightweight rods produced ‘acceptable’ augment, but one thing that was coming up at the end of steam (and as noted was probably a deciding factor in the relatively early scrapping of the Milwaukee F7s) was main pin failures. Other aspects connected with large two-cylinder power involved valve gear and main rod deflection (leading to buckling distortion or failure) and separation or breakage of piston rods (they are hollow in good lightweight practice). The incidence of these problems goes up geometrically with increased cyclic, and problems that are vanishingly slight at, say, 60 mph are clearly emergent at 80. Dividing the drive is already a significant improvement on, say, a 6000 hp locomotive that is intended to operate at 100 mph (see the NYC C1a design, for example) and while the conjugation appears to be gimcrackery much of the time, it is intended in particular to arrest high-speed slips (exacerbated by the unloading of short wheelbases by track imperfections and shocks) which can propagate to high speed when the locomotive has precise valve gear such as Franklin poppets.
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The most recent thinking is to underbalance