The Western Maryland Railroad pooled motive power with the Reading Railroad and when the Reading started using its new Class T-1s running times were improving on the Allentown, PA to Hagerstown, MD freight trains. This caught the attention of the Western Maryland and in 1947 it took delivery of twelve 4-8-4s from the Baldwin Locomotive Works. Being a "southern" railroad WM rejected the name "Northern" and called its 4-8-4s "Potomacs".

These twelve Class J-1 "Potomacs" (road numbers 1401 through 1412) were designed for freight service with 69" drivers, 26.5 x 32 cylinders, a boiler pressure of 225 psi, a weight of 506,500 pounds and a tractive effort of 70,600 pounds.

By 1954 all of these powerful locomotives were scrapped.