and I quote "The war emergency cars were a result of sheet steel shortages during World War II. An all-steel offset-side hopper car design had been proposed in 1935 and quickly became the standard 50-ton hopper. Thousands of these cars were built form the 1930's into the 1940's. By 1941 the AAR car committee developed a series of was emergency cars, which used wood in place of sheet steel....The cars were designed so the wood components could be easily replaced with steel sides after the war.

I'm lead to believe a lot of other cars were in the same game plan FYI...