In 1926 the Interstate Commerce Commission (ICC) was responsible for railroad safety. This function was a result of the Safety Appliance acts, which specifically made common carrier railroad safety a responsibility of the ICC. The Federal Railroad Administration was formed in 1966, and responsibility for railroad safety was eventually transferred to the FRA. The ICC was abolished about 1995.
The ICC rules were intended to assure that cars in interchange service would operate safely. Heavier freight trains made all wood cars less desirable from an operating standpoint; phasing out all wood cars was highly desirable after most freight cars had steel underframes and all steel cars were becoming common. So a date was set for ending construction of new all wood cars and for rebuilding all wood cars for common carrier service. This encouraged the transition to steel underframes, and eventually all steel cars, which were much better able to take the beating in heavy freight trains. There were probably also accounting reasons (tax code) for rebuilding cars before certain dates
John H. White's volume on the North American Freight Car in the wooden car era is very helpful as an overview of freight car technology up to the early steel cars.
CVM
-30-