The 2-bay covered hopper first entered service in the 1950s. The cars handle loads such as cement, sand and similar denser bulk commodities. In the 1960s, American Car and Foundry introduced the Centerflow design. The Centerflow relies on the car's rounded body to provide structural strength without the added weight of bracing, as well as making gravity unloading easier. This 2-bay car came in a 2,970-cubic foot capacity.
The first ACF covered hoppers were rib-side cars, like many others of the time. In the 1960's ACF eliminated the supporting ribs and started building what they called the Center Flow(R) hopper with a smooth, rounded side. The cars were built at several ACF plants, but the Huntington, West Virginia plant built only Center Flow cars since their inception and was the primary builder.