The Union Pacific was the only railroad to operate locomotives of the 4-12-2 wheel arrangement; consequently they were styled the "Union Pacific" type. Between 1926 and 1930 the American Locomotive Company delivered a total of 88 locomotives in the 9000 series, to which the railroad understandably assigned class "UP." Due to their long rigid wheelbase these three-cylinder giants were restricted to parts of the Union Pacific system not plagued by excessive curvature. But on the prairies west of Omaha, and elsewhere, they proved capable of hauling trains of up to 120 cars at 60 miles per hour. The center cylinder was angled at 9½ degrees above the plane of the axles. In these 4-12-2s, as originally delivered, its valve was controlled by a Gresley conjugated valve gear using a lever connected to extensions of the outer cylinder valves. Despite the usual maintenance complications associated with three-cylinder steam power, this arrangement worked remarkably well.
The original Gresley mechanism's bearings, however, were subject to excessive wear — a problem eventually corrected for most of the 4-12-2s by application of roller bearings to the gear. In the meantime, eight engines from the UP-1 and UP-2 classes of 1926 were retrofitted with a second Walschaerts gear and link on the right side which transmitted motion to the inside cylinder through a rocker arm. This device is visible on No. 9011, appearing above in a Charles E. Winters photo snapped at Cheyenne, Wyoming on August 5, 1946. No. 9011 is also distinctive in that, unlike the great majority of the 4-12-2s, its air pumps have been moved from the front of the smokebox to the sides of the locomotive. This alteration on several of the UP class produced a group Union Pacific enginemen referred to as the "Bald-Faced Nines."
As a class these 4-12-2s weighed 496,500 pounds; they had a boiler pressure of 220 p.s.i. and mustered a tractive effort of 96,646 pounds. They had 67-inch drivers, with the first axle offset by 18 inches to clear the third cylinder's rod driving the cranked second axle. The outer cylinder dimensions were 27x32 inches, but the inside cylinder had only a 31-inch stroke which obviated the need to set the crank pins of the three cylinders at other than 120 degrees. The grate area measured 108 square feet, the evaporative heating surface 5853 square feet, and the superheating surface 2560 square feet. No. 9000, the first of the class, is preserved at the Los Angeles County Fairplex in Pomona, California.