The J-1 was one of the last newly developed classes of steam locomotives of any US railroad. Inspiration came from experiences with the Reading T-1 4-8-4 that was also operated by the WM. So they ordered twelve 4-8-4 locomotives from Baldwin to be built in 1947. Designed for freight traffic, they had drivers with a diameter of only 69 inches. As the Western Maryland did not want to use the name “Northern” for this wheel arrangement, they called them “Potomacs”.

They were numbered 1401 to 1412. Being a modern design of the late forties, they had features like Timken roller bearings on all locomotive and tender axles, Boxpok drivers and integrally cast frame and cylinders. The boiler operated at 255 psi and had a firebox with four thermic syphons and a combustion chamber. With a cylinder diameter of 26.5 inches and a stroke of 32 inches, they developed more than 70,000 pounds of starting tractive effort. All were retired in May 1954 after a service life of less than nine years.