In general, all Class UP 4-12-2s were similar in their overall design and mechanics. The one exception began with Class UP-3, which included roller bearings, an ingenious design to late-era steam locomotives which greatly reduced wear and tear on the running gear. By the 1940s all of the Union Pacifics would feature this technological advancement. From an operational standpoint the 4-12-2s utilized Walschaert valve gear (initially, the first examples used Gresley valve gear), 67-inch drivers, tractive efforts exceeding 96,000 pounds, and were designed to burn coal. Somewhat curiously, for all of the trouble UP had with its 4-10-2 Overlands in attempting to maintain the complicated three-cylinder setup the railroad seemed more willing to deal with the issue in its 4-12-2s, perhaps because they did run quite well while in service.
By 1930 the railroad rostered 88 examples of the 4-12-2 among five different classes (Class UP-1 through UP-5) that were eventually numbered 9000-9087 in UP's roster. While most of the big freight haulers were relegated to the prairies two classes were used in the Pacific Northwest; Class UP-3 (#9700-9707) was assigned to subsidiary Oregon–Washington Railroad & Navigation Company (Oregon, Idaho, and Washington) and Class UP-5 (#9500-9514) operated on the Oregon Short Line (Utah, Idaho, Wyoming, and Oregon). Perhaps due to the complication of the third-cylinder or the fact that the experimental Baldwin 4-10-2 #60000, a compound-expansion locomotive, proved a failure no other railroads found interest in the 4-12-2 Union Pacific.
Of course, to operate a steamer with a such a long wheelbase required
long stretches of tangent, main line something not in great abundance
for many eastern lines (which also likely led to none purchasing either
the 4-10-2 or 4-12-2). In any event, despite the design's problems
Union Pacific remained happy with their fleet of 88 units for nearly 30
years. Finally, diesels began replacing the steamers starting in 1953
and all were retired or scrapped by 1956. Thankfully, UP's first
example, #9000, was preserved and today sits on display at the Los
Angeles County Fairplex in Pomona, California.
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