In 1958, some of the J's were used in local freight service after they were replaced on passenger trains by leased diesels.
The difference was the dog house on the tender and I have read the tandem rod was removed between the 2nd and third driver. Not all J's were modified. On page 70 of the Norforlk and Western Steam book, there is a Picture of the 600 class engine with the doghouse and extra tender after it was pulled from passenger service. The aux tender was in black paint only.
Maybe the N&W historical society could tell you more about the use of the J's late in 1958 or 59. I don't believe all of the J's were used for freight service after they were removed from passenger service.
All of my HO J class engines are the regular paint, not the excursion glossy paint. The 611 was modified with a double headlight in 1981 for excursion service, which was not done in regular service.
This is HO and using the J on local freight after a back shop visit would be OK since most railroads tested the locomotives on a short freight train before going back in the normal passenger pool after rebuilding.
As an example, there are pictures of SP GS4's just out of the back shop on a local freight.
BLI produced the passenger version only, not the late version with the dog house. You could certainly mount the dog house on the BLI version. I purchased an extra tender shell from the A class which had the dog house on it. The part could be removed and mounted on a J, but I like mine with passenger cars behind it.
CZ