A refrigerator car, also called a reefer, is a type of railroad freight car used to carry perishable foods at low temperatures, for preservation during shipment. The cooling of a refrigerator car can be done by ordinary ice, dry ice, or mechanical cooling systems. They differ from railroad cars that are insulated or ventilated, like those used to transport fruit, because those cars do not have a refrigeration system. There is a small amount of historical disagreement over who was the first to invent and use a refrigerator car, but it is known that they have been in use to one extent or another since the mid-1800s, long before domestic refrigeration as we know it was available.
Ice was the preferred method of cooling used in the refrigerator car for many decades after its initial use. Many different car designs were used as this technology evolved and improved. One method that soon became outdated was that of "top-icing," or placing a layer of ice in the top of a refrigerator car that carried agricultural products. The crushed ice added a great deal of weight to the car, and when it was found that the ice only maintained internal temperatures, rather than lowering them, this practice was largely discontinued.
Insulating a refrigerator car proved to be a technical challenge all its own. Inexpensive insulating materials tended to decay, causing them to lose their efficiency as well as tainting the car and its cargo with unpleasant smells. Higher quality insulators such as cork were too expensive to be cost effective. In these and other ways, refrigerated freight transport was an example of a great idea that couldn't quite be applied to its fullest extent for a long time, due to lack of technology.
After World War II, the availability of polystyrene foam and other efficient insulators represented a significant advance for the refrigerator car. The advent of evaporative cooling systems, much like our present air conditioners, meant that there was no longer a need for a virtual army of workers to re-ice the cargo every so often. This also cut down on costs in a significant way.
The use of refrigerated freight cars declined precipitously in the latter part of the 20th century, as the transportation of perishable goods began to shift from the rails to the highways. Transportation by 18-wheeled trucks became popular as mechanical refrigeration became more efficient and easier to incorporate into the trailers used to make over-the-road hauls. A shift back in the other direction began to occur in the 2000s, as high fuel prices made it more cost-effective to use railroad transport again.