As the Pennsylvania Railroad pushed west in 1850, it established an engine house at the base of the Allegheny Mountains where trains could be broken up or additional locomotives added to climb the heavy grades westward. This location now in the City of Altoona, PA. and the engine house with its small shop buildings grew into the largest complex of railroad shops in the world.

In 1866, locomotives were being built new in the Altoona Works in a shop known as the Altoona Machine Shop. The PRR built a total of 2,289 steam locomotives in this shop before it was converted into a locomotive repair shop in January, 1904.

A new locomotive erecting facility was opened in July, 1891 at the Altoona Works and for a while it was operated simultaneously with the Altoona Machine Shop. The new facility, known as the Juniata Shops, built a total of 4,584 new steam and electric locomotives until it ceased building locomotives in June, 1946. The last locomotive outshopped was a T-1, number 5524. The name Juniata was dropped in 1928, in favor of referring to the entire complex as the Altoona Works.

The first locomotive testing facility ever built went into operation at the Altoona Works. A unique locomotive test plant was designed by the PRR and it was operated as an exhibit at the Wolds Fair held in St Louis, Mo. After the Fair, in 1905, the plant was install in at the Altoona Works. This plant was a test stand that let the locomotive being tested run on rotating drums that could be regulated through braking to simulate the load of a train under various operating conditions.

At its high point, the Altoona Works consisted of four units: the 12 th Street Car Shop, the Altoona Car Shop, the Juniata Shops and the South Altoona Foundries. These four units were comprised of 122 buildings containing 37 acres of floor space, 4,500 machine tools, and 94 overhead cranes. The complex employed 13,000 people and the rail yard alone covered 218 acres.

Between April,1935 and June,1943, the Juniata Shops built a total of 124 GG1 locomotives. The following is a list of the GG1s built with the construction number and the date built.