Ten of these U-108b Class locomotives were built by the Baldwin Locomotive Works for the Union Railroad in 1936, the only 0-10-2s built new for a US railroad. They were consequently dubbed “Union” type locomotives.
The Union Railroad was created in 1896, running six miles from East Pittsburgh to Hays, PA. Between 1906 and 1915, the railroad expanded to include several other mills in the Mon Valley region, and was responsible for switching at each mill, delivering raw materials to the mills and finished products to interchange with the major railroads in the area.
After WWII, the Union dieselised and sold the Unions to the Duluth, Missabe & Iron Range in 1949. The DM&IR removed the engines' original boosters from the front tender truck, but compensated by adding weight to the locomotive frame. Most operated well into the 1950s, although #604 is the only survivor. It was donated to the City of Greenville, PA, in 1985 and is on display at the Greenville Railroad Museum Park on Main St.
As the steel industry declined in the US, the Union's operations were greatly scaled back, although it still serves three plants of the US Steel Mon Valley Works.