On the Pennsylvania Railroad, soon after World War II began, it became apparent that heavier and more powerful locomotives would be needed to handle the surge in traffic due to the war effort.

With war-time restrictions on new locomotive designs imposed by the War Production Board the PRR was forced to borrow a design from another railroad. It chose the C&O's Class T-1 "Texas" type and the C&O loaned it a locomotive for evaluation. PRR engineers prepared drawings based on the T-1 and the Altoona Shops built what would become North America's largest fleet of 125 "Texas" type locomotives.

The Altoona Shops started delivering the new 2-10-4s in late 1942. These locomotives were designated Class J1 and had 29" dia. x 34" stroke cylinders, 69" dia. drivers, a 270 psi boiler pressure with a resultant tractive effort of 95,100 lbs and each weighed 575,889 pounds. A total of 65 Class J1s were built and assigned road numbers 6450 through 6474 for a group of twenty-five delivered in 1942-1943, road numbers 6435 through 6449 for another fifteen outshopped in 1943 and road numbers 6150 through 6174 for the last batch of 25 delivered in 1943 - 1944.

In 1943, some changes were made to the design and 60 2-10-4 locomotives designated Class J1a were built along with the J1s. These locomotives were built in two batches numbered 6475 through 6500 and 6401 through 6434. The J1a "Texas" types were very similar to the J1s and had a weight of 579,975 pounds.

The J1 and J1a locomotives spent the war years hauling freight on the railroad west of Altoona. After the war they were displaced by diesels but the PRR found uses for them until they were retired in the late 1950s. There are no surviving examples of the PRR's 2-10-4s.