Q: Which products had their own freight fleet?

Anheuser-Busch owned the Manufacturers Railway in St. Louis, which owned reefers for shipping beer. They got out of the RR business in 2011.

Also, railroads routinely built fleets of cars for specific ladings, which they owned. B&O, for example had special cars for lumber, wallboard, autos and others that were visibly unique on the outside. They also had generic cars in restricted service, such a glass sand. You sometimes see RR-owned cars stenciled to return to a specific shipper.  I suspect more cars for specific products were RR-owned than by shipper or leasing companies.

I wish I could recall the publication, but I do remember that H.J. Heinz of Pittsburgh had its own fleet of freight cars.  In fact, they built them in their Northside facility in the city.  I always thought the vat cars (brine and pickles) were fictional, but actually did exist!

George

Milk cars also come to mind. "Milk Tank Cars" by General American Pfaudler were painted (or had metal signs applied) for local dairies.  The New England companies like Hood, Brookside, Whiting, are most recognizable but they were across the US.  Unfortunately, in today's modeling world of "lets get the most out of every production run" mentality, a lot of cars are painted up in a scheme that, while perhaps real, was probably never on that actual car type.  So it all depends on how prototypical you want to be.  The 3 rail hobby was never really a "proto-modeler" field but for a while there it got better. Now with all the custom runs for hobby shops and clubs, "fantasy schemes", etc. its back to a more toy like atmosphere...good or bad depending on your point of view...and requires more research if you want to be sure you are getting something reasonably close to prototype.  Or you can just roll with it and have fun.

In many cases, the private companies actually leased reefers from companies whose business was operating and leasing reefers.

"Billboard reefers" can be confusing nowadays to understand. Railroads and leasing companies did not rent out the sides of their cars for advertising. Rather, the private companies that had leased the reefers long-term would decorate them with large lettering and colorful graphics advertising what that company made - essentially, rolling billboards. This started in the 1890s, long before radio or TV advertising was possible.

As mentioned, problem with that was other companies wouldn't want to use those reefers. Railroads made more money shipping loaded freight cars than they did empties, but often they had to return a billboard reefer empty to the owner because no other company would use it.

The rule change in the 1930s gets pretty complicated I guess, but essentially it said that if you decorated your privately owned/leased car with lettering over a certain size (I think 16"?) then you had to be prepared to pay to ship it back to you empty at the higher rate it would have gotten when loaded. For some companies, that was OK and they paid it, but most changed the decoration.

However, this didn't affect railroad-owned cars, so about that time (late 1930s) they started adding large lettering and slogans advertising their passenger trains and service...particularly on the then-new 10'-6" "high car" boxcars.

For anyone interested in this subject, I suggest you get a copy of "Billboard Refrigerator Cars" by Hendrickson and Kaminski.