From the beginning of the American railroad industry in the 1830s, and continuing into the first decade of the 20th century, train passenger cars were constructed of wood. Builders had refined the craft of building such cars until their interiors and exteriors were objects of great pride and beauty. But wooden car bodies would easily crush and splinter in an accident or derailment, and the coal or wood stove used for heat could quickly turn a wrecked wooden car into a bonfire. Concerns for safety led to demands for metal cars. Near the end of the 19th century the railroads began building steel framed freight cars which could carry more weight, and thus earn more revenue. Those same railroads saw little economy in building steel or steel-framed passenger cars; such modification would result in heavier cars, and therefore more work for the locomotive.
In the first years of the 20th century, railroad companies finally gave in to the demands of safety. Steel had also become cheaper. Major railroads and car builders, such as Pullman, assigned engineers and designers to the task of fabricating a workable, safe, steel passenger car. For the next twenty years, the railroad industry settled on a design called the "Heavyweight." Typically, passenger cars in this period weighed about 80 tons, and were built on a substantial underframe of steel girders, which accounted for about 20% of their weight. Walls were a sandwich of sheet metal with heavy insulation filling. Because the great gaps for windows offered little structural support, steel beams were used. The floors were generally a thin layer of concrete spread on a wooden subfloor built over the steel girders. AT&SF #3355 is a classic heavyweight built in 1928 as a 70-foot long Chair Car, converted to a Snack Car in 1948, and until retirement transported passengers between Los Angeles and San Diego.