In my model railroad car movement system, icing is just a special instruction for reefers after they are loaded . The loading industry normally filled the ice bins when the car was loaded. The railroad just topped up the ice as needed as long as the car stayed on its trackage. Reefers had priority for delivery to online industries and to be placed on the interchange if going offline. The problem with making it a commodity is the reefer doesn't consume the ice as a real reefer would as the ice melts.