The PRR J1 was a class of 2-10-4 "Texas" type steam locomotives built between 1942 and 1944. The J1 had over 95,000 pounds-force (422.6 kN) of tractive effort, plus an additional 15,000 lbf (66.7 kN) if the booster engine was used.

As with many of the Pennsylvania Railroad's steam locomotives, the J1 had its headlight above the smokebox. Like the M1 the J1 had a keystone numberplate, unlike the round numberplates seen on the rest of the PRR's freight steam locomotives. Wartime restrictions forbid the design of a completely new engine so the PRR basically adopted the C&O design almost without change. As a result, they were equipped with Baker valve gear instead of Walschaerts valve gear which was more common on the PRR. Additionally, they had radial-stay fireboxes instead of the Belpaire fireboxes seen on nearly all of the Pennsylvania Railroad's steam locomotives. Mechanically, these locomotives were identical to the C&O's T-1 class 2-10-4s. As initially built, the middle driver was blind to facilitate tracking on curves. With experience the railroad determined that this wasn't necessary and after shopping the middle driver was equipped with flanges. Other changes to the driving wheels included counterbalancing, as it was found in service that the J1s ran roughly like their T-1 brethren. This was also fixed in later shopping, also increasing the driver diameter to 70”. as it was The engine did have lateral motion devices to allow some sideways drive axle travel which did enable it to work on Pennsy curves. Other PRR changes included the curved front side cab windows, and the cast pilot with drop coupler.

During World War II, the PRR needed heavier locomotives to pull freight and military equipment, but wartime restrictions prohibited the development of a new locomotive design. In response to this the Pennsylvania Railroad borrowed a 2-6-6-4 Class A of the Norfolk and Western Railway and a 2-10-4 from the Chesapeake and Ohio Railroad. Both locomotives underwent extensive testing, with the C&O 2-10-4 chosen to be produced. A total of 125 were built at PRR's shops in Juniata, Pennsylvania. They came to be known as the PRR's "War Babies," but the J1's remained in service into the 1950s. When the Pennsylvania Railroad converted from steam power to diesel, 100 were scrapped in 1958 and the remaining 24 were scrapped the next year. One locomotive No. 6435 is rumored to be buried in a previously PRR owned yard, now covered by a Norfolk Southern mainline and a road bridge, but this hasn't been confirmed with an additional bell being preserved. ^{[citation needed]}

• J1 class diagram