my guess is that the three compartment tank car contained refinery products used in smaller quantities like lube oils.
everywhere that I've lived there were tank farms by water. for a couple of years I worked next to a tank farm located on the Hudson river. I assumed the tanks contained home heating oil, diesel fuel and gasoline. the petroleum products were delivered by a ship.
when the water was frozen, were these tanks resupplied by railroad tank cars?
I once supervised loaded a tank car with an expensive chemical used to make mosquito repellent. Considering the difficulty in filling the car with liquid I'd say many tank cars were dedicated to one load. Tank cars used for transporting crude oil might be blown out with steam and used for fuel oil, maybe gasoline or kerosene.
in a similar vein hopper car loads sometimes got wet and froze. when thawed was coal added to the huge coal towers where it might refreeze?