Since articulated freight locomotives in the 1920s did not yet reach the same speeds as ten-coupled locomotives, Union Pacific had the class 9000 developed with six coupled axles in one frame. The unique wheel arrangement with a four-wheel leading bogie, six sets of drivers and a two-wheel trailing truck was called “Union Pacific” by the UP. The 1926 prototype was officially given the class designation UP-1, while further batches were designated UP-2 to UP-5.

Only the fourth driving axle had no wheel flanges, but the first and sixth could be moved laterally by two inches. In order to be able to use the steam from the large boiler, three cylinders were installed. The middle one was offset upwards at an angle and was controlled by the outer cylinders via a Gresley valve gear.

In addition to the UP, two subsidiaries also received additional locomotives of this type, so that the total number ultimately reached 88. These were eight on the Oregon–Washington Railroad and Navigation and 15 on the Oregon Short Line Railroad. Although they were only intended for speeds of 35 mph (56 km/h), they often reached 45 to 50 mph with freight trains and could easily reach 60 mph (97 km/h) while running steadily.

Their area of operations focused on Nebraska, where they increased the speed of freight trains in the flat land. However, it became apparent that the Gresley valve gear's friction bearings were subject to high levels of wear. Only in eight of the oldest locomotives was this replaced with a Walschaerts valve gear and in the machines built from 1928 the Gresley valve gear was given roller bearings.

During the Second World War they had to work hard and were no longer adequately maintained. This meant that some were set aside during the war. The entire class was withdrawn from service between 1953 and 1956. The only surviving locomotive is the prototype number 9000. It is not operational and belongs to the Railway & Locomotive Historical Society in Pomona, California.