I have uploaded some documents found at the Minnesota Historical Society in the Northern Pacific Co files.
These are AAR documents and the desk staff on site OK'd these of any copyright restrictions on the MHS's part as he felt these were originally for open distribution from the AAR and so they are posted here.
The orders covering the substitution of refrigerator cars for box cars are listed from inception to about 1956. No info past that date was present.
These 10 pages in the folder list the history of substitution arrangement - Refrigerator Cars for Box Cars on Westbound Continental Traffic, and are found in the file: Refrigerator Cars for Box Cars
James Dick - Roseville, MN