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. Heavyweights are said to offer a more luxurious ride due to their added mass (from the plate steel construction and concrete floor) and, usually, six-wheeled trucks (bogies). The stepped roof line of early heavyweights usually consisted of a center sill section (the clerestory) that ran the length of the car and extended above the roof sides by as much as a foot. This section of the roof usually had windows or shutters that could be opened for ventilation while the train was in motion. However, railroad crews and passengers quickly discovered that when these windows were opened on a passenger train pulled by one or more steam locomotives, smoke and soot from the locomotives tended to drift in through the windows, especially when the train went through a tunnel.
In the early 20th century, air conditioning was added to heavyweight cars for the first time. An air conditioned heavyweight car could be spotted easily since the area where the roof vent windows existed was now covered, either partially or in full, by the AC duct. As lightweight cars were introduced, many heavyweight cars were repurposed into maintenance of way service by the railroads that owned them.
Lightweight passenger cars required developments in steel processing that weren't available until the 1920s and 1930s. By building passenger cars out of steel instead of wood, the manufacturers were able to build lighter weight cars with smooth or fluted sides and smooth roof lines.
Steel cars were ushered in at the beginning of the streamline era of the 1930s (although not all lightweight cars were streamlined) and steel has continued in use ever since then. With the use of steel for the car sides, railroads were able to offer more innovative passenger car types. It wasn't until after the first lightweight cars were introduced that railroads began building and using dome cars because the sides of heavyweight cars weren't strong enough to support the weight of the dome and its passengers. Lightweight cars also enabled the railroads to operate longer passenger trains; the reduced car weight meant that more passengers could be carried in a greater number of cars with the same locomotives. The cost savings in hauling capacity coupled with the increased car type options led to the quick replacement of heavyweight cars with lightweight cars.
Courtesy of Wikipedia
