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Interesting question with a seemingly wide scope so here goes.
During the war years, the railroads needed every piece of equipment to handle shipping needs of the times. During the 1930s, the economic depression reduced equipment demands but things changed in the late 1930s. There were many woodsided box cars . refrigerator cars , Gondolas and during the war years Hoppers with steel framework but having wood sides. Many of these cars were rebuilt and modernized in this period to extend their life. A lot of these cars lasted in service into the 1950s.
World war 2 brought a rize in the number of tank cars on the rails because of German submarines operating off the east coast sinking tankers . Tank cars reduced this danger to the supply.
Steel cars in the USA came about in the late 1920's early 1930s . The Pennsylvania RR's x-29 box car design was born in this time and was used bu several eastern RRs as well as the PRR.The AAr created a steel box car design about 1937 which sort of became a standard at that time.
Reefers at the end of World war 1 were usually 36 foot wood car designs with ice bunker cooling. Some railroads and private owners had insulated cars for the shipment of milk and dairy products , the milk sent in both tanks and individual tanks. In the 1920s, 40 foot designs came about also wood Construction. Steel reefers came about in the 1930s.
In the 1930s , the ICC ruled that use of advertising on reefers , the so called Billboard cars was not allowed unless the car was carrying the product . Meat reefers continued with the owners logo and trademarks since that was what they were carrying. The 36 foot wood reefer continued in service into the 1960s because many of these cars had overhead trolley fixtures to hang the sides of beef etc which matched the facilities at the meat packing plants where they were unloaded. Some of these plants had sidings in parallel and cars would be unloaded by setting up the trolley to roll the side of beef through adjacent cars to reach the packing plant unloading dock.
In the post war period, 50 foot box cars . reefers, hoppers , gondolas stated appearing on American railroads as wood cars were being phased out. Welded tank cars and frameless designs began to appear in the late 1940s.i
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