Regarding the LFM - UNIVERSAL center driver, it would appear as an attempt to deal with the inherent trouble with 5 coupled axle designs and 63 inch drive wheels common to the Texas & Pacific 600 2-10-4. The balance issues of these small wheels were manifold. Alfred Bruce has the photo of SP 5000 the prototype engine showing a spoked center driver with a truely massive half circle counterweight. It would be easy to assume the LFM - UNIVERSAL driver is an addition in an effort to increase the low operational speed designed into SP 5021 or was it rather really just a drive wheel structural problem. The SP 5000 engines were three cylinder locomotives so the addition of the UNIVERSAL drive setup with three rods would not have been to "dynamically balance" the engine - three cylinder drive was already fairly well balanced.
Thereupon hangs the tale. The 5000s had divided drive, with the inside cylinder angled down so the main would clear the first axle and driving on a cranked second axle using lower stroke (28" vs 32") So there is less need for rotating or recip balance in the outside mains ... but it and cross balance will be at weird angles. So I suspect that the large outside counterweights are more large cored areas in the driver castings, with some of the internal space not actually weighted. It's the rod inertia, not the 'balancing', that causes most of the pin and bushing problems.
Bruce said (p 301) "... the use of three cylinders permitted the power to be delivered directly to two driving axles with good balancing conditions in the 63-in driving wheels. The outside piston and main rods were unusually long but produced no ill effects".
We should keep in mind here that this is with 25" cylinders vs the 27" for the Nines,with comparatively low pressure (225#) so the weight of the longer rods was unlikely to be "that much" of an issue. If anything it would give better rod angularity.
"... the engines performed well at operating speeds of from 30 to 35 miles per hour with the 63-in drivers ... Long continuous operation at higher speeds ... increased the maintenance, as might be expected with the heavy middle-engine moving parts."
I am not entirely sure why the middle-engine parts were particularly 'heavy' especially at the reduced stroke and presumably shorter main length, but we do learn that there were problems with the 'floating bushings' in the center big ends ... perhaps in part due to the same sort of issues with Gresley gear that produced big-end trouble in the LNER A4s. Apparently when the center gear started to get out of tram the result was 'breaking crockery' in lineside houses... probably greatly magnified at the higher rotational speed necessitated by the 63" drivers. The Nines had only 4" more but it's right in the critical range for effective balancing. (As Bruce also points out p.302 the UP Nines were intended as a 35mph engine, but the characteristics of the long wheelbase and three-cylinder drive allowed them to run considerably faster in practice -- sensibly or not!)
It seems nominally sensible to me that if the initial T&P 2-10-4s, with abbreviated frame and no effective lateral chassis control at the rear from the truck, and only two cylinders, were stable at 45mph after getting better balancing in 1938, the situation on a three-cylinder locomotive with a four-wheel lead truck could only be better. Provided, that is, that all the weird science involved with keeping the engine's inside valve set and the inside rod maintained was properly conducted... there may be relevance in the fact that UP rebuilt its 4-10-2s (with great effort!) into 2-cylinder locomotives of doubtful balance effectiveness, despite having a large dollop of distinctive competence in care and feeding of three-cylinder power via the Nines, but SP kept theirs 'three barrels of steam' to the end.
Something else I do not know is whether the same mindset, or people, involved in getting the 600s rebalanced so carefully would have been applied to an engine from a different builder, with nominally smoother drive but heavier maintenance requirements. I find it more than likely that someone did some thinking at both SP and LFM -- the latter proud enough of their careful engineering of driver casting holes to match anticipated stresses that they mentioned the point in their advertising in the '47 Cyc -- and that a little forensics at Pomona compared with build specs might tell us more about the changes that were made, and whether the result would be as comfortable at higher speed as the T&P engine was ... comfortable, that is, until accumulated play began to show its effects...
Meanwhile as confusedly quoted on the Web
they were too rigid for the curves snaking through the Donner Pass, where they were first tried. After relocation to the Sunset Route east of Los Angeles, the 4-10-2s ran until the mid-1950s. It is there - between Roseville and Summit, Calif on a 2 1/2%, 80-mile grade -- Bruce says, "...the engines were remarkably successful because the operating condtions prevented excessive speeds and the even torque of the three cylinders prevented undue stalling at low speeds."
This was evidently written by somebody unfamiliar with Southern Pacific or Southern California, or who had some unrecognized trouble with drag 'n drop text editing. Bruce was writing about the initial use of the 4-10-2s on Donner, and it is not he but George Drury who noted that the 4-10-2s were more successful in the South (they ran well into the '50s, and 5021 went to the Fairgrounds under her own steam). I have actually read Boynton's Three Barrels of Steam but a long time ago, and don't remember seeing the point of rebuilding or even faster operation being discussed at all (the book dates from 1973 and is not really geared to tech-heads).
We're straying too far from the original thread premise ("too many legs and not enough steam" via do-it-yourself kits) and should probably start a separate topic on 10-coupled non-articulated engines if we want to keep going with this particular sort of bone in our teeth.