The cylindrical covered hoppers were really the first step in the evolution of ACF's centerflow hopper design - one of the most enduring on the railroad scene. Like the conventional cars which preceded it, and the smooth-sided cars which followed, cylindrical were made available in several sizes for different lading. Smaller cars are used for cement, sand, and other dense materials. Larger cars were employed for plastics, phosphates, and other very light loads. Perhaps the most common cars are the three and four bay mid-range cars used for grain service.