The design and spacing of the ore dock hoppers was established about 170 years ago. The ore dock design drives the design of the ore car. No railroad or shipper would use an ore car if they did not have to. It is an obsolete design driven by the ore dock design. A modern car would be a larger volume if it did not have to unload on a ore dock. The only commodity that would be carried in an ore car is something that was going to be unloaded at an ore dock to be load into an ore ship. I believe that the iron ore products loaded into ore cars runs from reddish brown to almost black. Coal would never be loaded into an ore car as it is fairly light. Coal cars are usually in the 4000 cuft size car.
An excellent observation. Also please remember just how REALLY heavy that iron ore, or Taconite, load is. Those "little" ore cars weigh about 100 tons when loaded. Thus, loading iron ore or Taconite into a "normal" two bay, three bay, or four bay coal caring hopper car would seriously exceed the weight capacity of such a car.