While not the most common rail cars (especially now), box cars are probably the most versatile, with the ability to carry just about anything that can be loaded into it. Originally they were hand loaded, but of course forklifts and other devices have sped up the process. However, box cars are still a good deal slower to load and unload than other types of car, which helps to explain the decline in their use since World War II. They've been largely supplanted by intermodal containers, which can be loaded once at the factory and unloaded once at their final destination.
This happens to be an old Oscar Mayer car. That's right, the hot dog and baloney people. A lot of companies had their own fleets of box cars, which sped up the shipping process a bit and also provided a nice rolling billboard.