Now, that's interesting, according to Robert Church (Southern Pacific Ten Coupled), the 4-10-2's were initially designed by Alco who then finalised details with Russell? from the SP.
SP signed for 16 engines on Oct 17 1924 with construction starting six days later.
Fetters could not get the UP board to agree to the new three cylinder arrangement, documentation has it that Fetters was already looking ahead at the 4-12-2 design and the possible use of three cylinders around the same time.
Alco seized the moment and offered to build UP 8000 as a test engine; basically free of charge unless it was successful. UP agreed and ordered on 14th Nov 1924.
Interestingly, 8000 was not built at the same plant as the SP order (which had already begun at the Schenectady) but at the Dunkirk NY plant.
Kratville and Church were life long friends, Kratville being Church's mentor in his early years chasing steam and published Church's first book on SP engines. His books are the equal of Kratville and a must on any SP steam modellers shelf.
I was just down the road from Pomona on my last vacation, but didn't take the time to visit, shameful really but the lure of ACe's and GEVO's was too strong
I did a quick trawl today, and yes there were brass models, more to the point, all the web pages I found were already book marked in my browser, I'd just filed them somewhere important that I'd forgotten about
Utah Railways also ran a handful of TTT engines which were very close to the UP design, certainly had the tell tale UP style cab with angled front faces, I'd need to dig a bit deeper to see if they were linked to the UP orders.
I do enjoy all this research malarky.