The Butler and Pittsburgh Railroad Company incorporated April 8, 1896 and completed, spectacularly, by October 27, 1897 including a long, single track bridge across the Allegheny River.

Also in 1897, PS&LE and B&P were consolidated into the Pittsburgh, Bessemer & Lake Erie under majority ownership of Carnegie.

The Union was responsible for the various switching task within each mill, for delivering raw materials to each mill (which would arrive on the Union via interchange with the Bessemer & Lake Erie, another US Steel owned and operated railroad) and for delivering the finished products to interchange with the major railroads in the area (most notable the Pennsylvania, the Baltimore & Ohio and the Pittsburgh & Lake Erie).

In 1906, B&LE leased, and later sold, to Union Railroad the portion of line between North Bessemer and East Pittsburgh.

Unlike the rest of the steel industry, the Union was relatively accepting of modernization as demonstrated by construction of a then state-of-the-art yard and dispatching center in Duquesne, Pennsylvania, in the early 1950s.

With the decline of the steel industry in the United States, the Union's operations were greatly scaled back.

For inner-mill service the Edgar Thomson plant uses US Steel own EMD-switchers to move the hot metal subs and for tressel unloading.

After closing the Riverton bridge in 2008[citation needed] there is no rail connection between the URR network and Mckeesport Tubular Operations "Camp Hill".

Union Railroad continues to serve the Mon Valley and have since expanded its customer base to include Dura-Bond pipe coating in the former Duquesne Works site and General Electric in West Mifflin hauling special oversize generators.

This unique combination in addition to the steep grades around Pittsburgh demanded some special tractive force.

The 0-10-2 wheel arrangement was named the Union type after the railroad and produced over 100,000 pounds of tractive effort.

Only one survives today and is on static display in Greenville, Pennsylvania, painted as Duluth, Missabe and Iron Range (DM&IR) #604.