Early passenger cars were constructed from wood; in the 1900s construction shifted to steel and later aluminum for improved strength. Passenger cars have increased greatly in size from their earliest versions, with modern bi-level passenger cars capable of carrying over 100 passengers.

Heavyweight Vs. Lightweight. A heavyweight car is one that is physically heavier than a lightweight car due to its construction. While early cars used wood construction, Pullman switched to heavyweight riveted steel construction in 1910, more or less at the same time as other rail car manufacturers.

The best years for Pullman were the mid-1920s. In 1925, the fleet grew to 9800 cars. Twenty-eight thousand conductors and twelve thousand porters were employed by the Pullman Co. [14] Pullman built its last standard heavyweight sleeping car in February 1931.

The use of steel gave rise to "heavyweight" cars comprised entirely from metal, hence their name. Steel not only increased strength but also reduced the chance of fire. The first use of an all-metal car was theorized as early as 1846 by H.L. Lewis to be built from wrought iron.