To meet the increased demand for heavy freight locomotives during the Second World War, the PRR turned to the C&O and used their T-1 as the basis for their J1. Because of this origin, they were given the nickname “War Babies” and were ultimately the largest 2-10-4 “Texas” class in terms of numbers with 125. They were built at the PRR's Juniata Works in Altoona.

With the booster on the trailing axle they achieved a starting tractive effort of 110,100 pounds (490 kN), while the boiler output of 5,644 hp was sufficient for higher speeds with heavy freight trains. Some of the locomotives had a frame cast in one piece. Depending on the source, these were the 60 members of subclass J1a.

A teething problem of the J1 was its rough running, which apparently arose from a mistake when taking over the blueprints from the C&O. This was soon fixed and at the same time the drivers were enlarged from 69 to 70 inches. With the switch to diesel, they became dispensable in their original role in the late 1940s. However, the PRR also found enough other tasks for them so that they were only retired between 1957 and 1959.