Ore cars are a type of hopper car, but they have a dedicated service. Whereas generic hopper cars can be used to carry a variety of materials, ore cars are shorter because they are designed to carry the much heavier iron ore (as opposed to the lighter coal carried by larger hopper cars). Ore cars are also known as "jennies". They were often seen in unit trains, being shuttled between ore docks and steel mills. After WWII, ore cars carried mostly taconite. They could also, occassionally, be found carrying sand or gravel.

Below are links to the various reports that show all of the S-scale ore cars ever produced. Click whichever report gets you to the information that wish to find.
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These definitions, in alphabetical order, may clarify some of the terms used in the reports linked to above:
Iron Ore Rail Cars: A History Of The Classic Jennie
CN Buys Ore Cars (2009)