A
refrigerator car (or
"reefer") is a
refrigerated boxcar (U.S.), a piece of
railroad rolling stock designed to carry perishable freight at specific temperatures. Refrigerator cars differ from simple
insulated boxcars and
ventilated boxcars (commonly used for transporting
fruit), neither of which are fitted with cooling apparatus. Reefers can be
ice-
cooled, come equipped with any one of a variety of mechanical refrigeration systems, or utilize
carbon dioxide (either as
dry ice, or in liquid form) as a cooling agent.
Milk cars (and other types of "express" reefers) may or may not include a cooling system, but are equipped with high-speed
trucks and other modifications that allow them to travel with
passenger trains.