Lima 4-8-6 - Trains Magazine

ShroomZed

You know, was there ever a locomotive built with 'Unitary Machine Support' as depicted in the proposal? 

None that I know of; the principal advantage of two 'huge' inside cylinders was much better achieved, and of course much much more easy to maintain, through simple outside rod connections with better balancing (post-Eksergian) and lightweight 'high-dynamic' rod steels and detail design.

Part of the story also involved the general turn away from higher and higher single thrust pressures, together with the absolute peak in usable boiler pressure in a staybolted firebox that was somewhere between 300 and 315psi, combined with the perceived need to have 'higher' road speeds than even a somewhat 'optimized' CMS locomotive would provide.  The evolving issue is the water rate associated with the 'achievable' single-unit horsepower (which, remember, is a function of speed; it can be thought of as a measure of drawbar TE at speed).  By the time you get into what might be available with big cylinders (with lightweight pistons, hollow rods, and all that jazz) at high pressure, you either run into impossible cutoff issues, or the need for some kind of staged steam admission, or such a heroic mass flow (and hence water rate) that you need an appreciable part of the TE just to pull the water you need.  Consider that the PRR Q2 had a higher sustainable horsepower than any contemporary articulated locomotive, and that out of only five driver pairs... and a thirst to match.

Better to divide the cylinder capacity into a couple of engines in an articulated chassis, once you've solved the stability problems (which Baldwin started in 1930, N&W finished by 1934, and Alco commercialized widely starting a couple of years after that) and have, say, the D&H's conclusive and final answer to the high-pressure experiments: its Challengers.

Or divide heavy 2-cylinder thrust into a duplex configuration (and, if you are wise, conjugate it) if you don't want the high overall length.  

What you don't see is anyone attempting to merge the joy that is an AMC Berkshire with the joy that is a UP Nine; that is, attempting to make a 'Berk-and-a-half' by extending something like the successful C&O T-class 2-10-4 into a 2-12-x.  Either with two big inboard cylinders or three balanced ones.  And I think there are a fairly wide variety of reasons, mostly NOT accidents of timing or circumstance, that account for that.

Frankly it is difficult to imagine any twelve-coupled simple 2-cylinder-quartered engine much 'better' than something the size of a N&W A class (for the sane) or an Allegheny (for those who can pay for high-mass overcomplexity).