The J’s were built for passenger duty, but used for freight after passenger service was discontinued. The J’s were some of the finest locomotives ever produced, so it would really have been a waste to just scrap them after passenger service died off. Nearly every railroad that ran passenger service had relatively new locomotives when passenger service was done away with, so they used their locos for freight until they were replaced.
The N&W didn’t discontinue passenger service, they dieselized it, at first with leased ACL E units, and then with their own fleet of steam generator equipped GP9s (or GP18s).
trying to track down a photo, its in the N&W Giant of steam book, looks pretty much like a Santa Fe 4-8-4 with the J siderods and looks, but no streamlined sheeting, its an addon over the standard loco.
They were built during the war, and there were wartime restrictions of materials, and the N&W had to report to the War Emergency board about their construction details,
and were desiganted General Service-freight and passenger.
Streamlining got applied to them after the war.
The larger siderods proved hard on the bearings and the war production board allowed N&W to replace the rods and add the streamlining in 1944.
The wartime J’s originally were slated just for passenger service, because the N&W had a large increase in passenger requirements during the war (Hampton Roads had the largest concentration of military bases anywhere), but because of the lack of reliability due to the bearing failures they were bumped to freight, and they ran a lot of larger sections using Class A’s for motive power. The Class K’s were also used almost exclusively on passenger consists.