Final answer:
Ice tanks are positioned at the top of refrigerated railroad cars to enable cold air to circulate downward, as cold air is denser and sinks, providing an even distribution of cool temperature to preserve the car's contents.
Step-by-step explanation:
Ice tanks are located at the top of the refrigerated railroad car because of the properties of thermal convection. Cold air is denser than warm air and thus tends to descend, while warm air, being less dense, rises. Therefore, placing ice at the top allows the cold air to circulate down and throughout the car, ensuring an evenly distributed cooling effect. This method mimics early icebox designs and effectively utilizes the thermal properties of ice and cold air to keep the contents of the railroad car cold and preserve the items being transported, such as farm commodities that were sensitive to temperature. Refrigerated rail cars were pivotal in transforming long-distance transportation of perishable goods, enabling produce to be shipped safely all over the world, which maximized profits for farmers and changed how distance was factored into logistics, as indicated by the Von Thünen Model.