Alan:
The cars are short. They were designed to carry iron ore in Northern Minnesota to the docks on Lake Superior. Iron ore is more than 2-3 times as heavy as coal ( I don't have my mineralogy books here at home to check the exact amount) , therefore the cars are significantly shorter than the standard hopper car. The iron ore was shipped from Duluth and other ports on Lake Superior to ports on Lake Erie (Toledo and Cleveland, Ohio). There other railroads would transfer the ore to the steel mills. The B&O was one of these railroads, they could only fill the standard hopper by about 1/3 rd of the normal level of coal. Unfortunately, USAT chose many railroad logos for their cars and missed the few that actually used these cars. My memory may be a little off, but of the railroads shown by USAT only the Great Northern actually used these cars. The major user of these cars was the Duluth Missabe and Iron Range, sadly missing from the cars available.
Chuck N