My guess is they were handling bulk petroleum products (gasoline, diesel, lube oil, etc.) to various oil company bulk plants. This was a pretty normal method of distribution from refinery to local distributor in those days. I seem to remember pix of Conoco multi-domed cars, which indicates they had several seperate compartments for handling several commodities at the same time. However, many of these commodities could have used cars very much like the crude oil cars. This reflects educated guesses, and perhaps someone has some more factual information.