Monon 4-10-4

The SP and the UP ran 3-cylinder 4-10-2's. The flat "chin" at the bottom of the smokebox and a below-center headlight were features of UP 4-10-2's.

A 4-10-4 isn't that far removed from reality. Pennsy Q2 4-4-6-4 Duplexes had rigid frames. They were pretty close to 4-10-4's. Articulated frames and another driving axle on their front engines would have made them excellent Challengers, IMHO.

Looks like one of Dr. Richard Leonard's creations.  I'd guess the base locomotive was an SP/UP 3-cylinder 4-10-2, with the firebox, cab and tender from a C&O 2-10-4.  The stack looks a bit small but it is still the characteristic capped  type favored by the railroad.  There was such a locomotive proposed, and the specs can be found in George Hilton's book on the Monon.  Dr. Leonard appears to have incorporated mostl of the major features from the specs.

Thanks guys, interesting comments. At first glance this "4-10-4" seems to have kinda small side rods, but we have to remember there's three sets. If I remember correctly, this tentative plan eventually evolved into plans for the Challenger 4-6-6-4 articulateds. And yes, the Pennsy Duplexes make an interesting comparison.

Ace, you are correct.

Between 1926 and 1930, the UP acquired 88 4-12-2's. They promised to haul freight faster and more economically than other locomotives then available. The first, No. 9000 (now at Pamona), "bore out the theoretical curves." She ran as well (or better) than her designers had hoped.

As more 4-12-2's were put to work, maintenance costs mounted. The center main rod and bearing on a cranked driving axle were sandwiched between the frame. It was difficult to reach and even harder to service. The best man to climb in there was a short, beefy guy "built like a bull." The UP hoped that training crews to operate these locomotives well would keep maintenance costs down.

Then the Great Depression hit. Eighty-eight heavy freight locomotives could not be retired or replaced. Solid bearings in the Gresley valve gear put valve events "out of time" as they wore, and that stressed the outer cylinders, running gear and frames. Roller bearings would have helped, but the 4-12-2's didn't get them. Throwing good money after bad, I suppose.

William Kratville and John Bush wrote a 2-volume history of the 4-12'2's, THE UNION PACIFIC TYPE, printed by Barnhart Press in Omaha. They included drawings of a proposed conversion of 4-12-2's to 4-6-6-2's. That was turned down in favor of early and later Challengers.

Overseas, 3-cylinder and 4-cylinder locomotives ran long and well. Some are still running in excursion service. My guess is that American 3-cylinder locomotives packed more power and weight. They exceeded mechanical limits of the third cylinders and running gear and became maintenance headaches.

Just my two cents. I'm enjoying this thread. I like talking about what might have been in modern steam power.

THe Pennsy Q2 could have been a 4-10-4 with one small mod.  And that would be cranking the second and third axles for a pair of internal connecting rods.  Such a  mod. would have eliminated the slipperyness associated with the Q2, and allowed for unprecedented smoothness in a ten coupled loco.  Stealing a page from Bill Withun, we could then install high speed compounding in such a chooch. With all that, we'd have a pretty decent late model Pennsy steamer...if you could get the boiler to quit leaking !