I stumbled on these pictures of ice refrigerated freight cars taken in the 1940's on the Library of Congress web-site. From what I gather, the cars were pre-chilled with cold air, the the wall containers were loaded with block ice. The refrigeration plant was ammonia.

A little bit of our history, hope someone else enjoys them.

1) San Bernardino, California. Icing refrigerator cars at the ice plant

2) San Bernardino, California. Cars being precooled at the ice plant. Air at a temperature of twenty degrees Fahrenheit is blown through the cars for twenty minutes in one direction, then in the other. Shippers specify the number of hours precooling required for their product

3) San Bernardino, California. A tunnel at the ice plant through which air is blown on its way to the refrigerator cars. Cooling tubes along the walls keep the air at a constant temperature

4) San Bernardino, California. Icing Atchison, Topeka and Santa Fe Railroad refrigerator cars at the ice plant

5)San Bernardino, California. Icing Atchison, Topeka and Santa Fe Railroad refrigerator cars at the ice plant