These big National Steel Car Plastic Pellet hoppers are built especially for hauling plastic pellets from manufacturers to molding plants. As with similar lightweight materials, bigger cars are used to carry a maximum payload, and hoppers like this can be found at producing and consuming industries.
 
These ready-to-run cars have the feature you want: separate roofwalks, pneumatic piping, control valves, brake cylinders, brake wheel, air reservoir and hatches, wire grab irons, and working knuckle couplers.