Here is a cover letter and the first two, of sixteen, pages from the August, 1949 Chicago Association of Commerce and Industry “Package Car Service” Bulletin in the SRHA archives.
The term “package car” appears to have been used, at least in the Eastern US, almost universally for shipments that were not using an LCL tariff for loads going to more than one destination or consignee. Pages two through sixteen show package car routes out of Chicago to locations in the US and Canada.
Although I cannot locate a note confirming, the first entry for a route in “VIA” column is obviously the originating Chicago railroad.
There are multiple copies of the “Bulletin" in the SRHA archives in addition to several version of the “Merchandise Car Directory” published by the Southern Railway. I expect all major railroads had similar lists or publications. An article on package cars will be in an upcoming issue of “TIES”, the SRHA magazine.
The Southern chose not to have box cars marked for the service. As Atlanta was the largest source of out bound cars, the logic was there would always be empties in Atlanta to load at the three freight stations (plus Sears Roebuck) so having specially painted cars return empty was not efficient.