These cars were sold in 4 and 12 car sets (as well as singly). It was variously available in kit form or ready to run.
I'm not sure what specific prototype these models might be based on (if any), but they look a bit like the 8,000 gallon tank cars built by NACC in the late 60s / early 70s. Shorty tank cars such as these are used to haul high density liquids (since a longer car loaded with said materials would only be half filled before exceeding its weight capacity).