First introduced by the Baltimore & Ohio railroad in 1830, a gondola is typically an open-topped railroad car used for transporting loose bulk materials that can handle being exposed to the weather and are not likely to be stolen while in transit. Covered gondolas exist for those materials that need to be protected from the weather. Because of their low side walls, gondolas are also suitable for carrying steel plates, coils, or bulky items such as pre-fabricated sections of rail track and pipes. Gondolas are distinct from hopper cars in that they do not have angled doors protruding from their floor to empty their cargo. The Union Pacific web site states that if you chop a box car in half horizontally, you have a gondola. What we would call a regular gondola is one which has sides that are typically 5 feet tall. Taller versions are called "high side" and shorter versions are called "low side". At a high level, there, basically, have been five types of gondolas produced over the decades. Initially, these were simply converted flat cars (wooden sides and ends bolted to existing flat cars; kind of like a sandbox on top of a flat car). Then there were wood-sided cars, initially on wood frames with truss rods, and later with steel frames. During WWII the composite car was built using steel for the frame and the bracing, but wood to fill the panels themselves. The final and current version is the all steel car (some have aluminum bodies to reduce their empty weight).

Below are links to the various reports that show all of the S-scale gondolas ever produced. Click whichever report gets you to the information that wish to find. The bottom of this page has additional references that might be of interest.
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These definitions, in alphabetical order, may clarify some of the terms used in the reports linked to above:
Click the red header text for the external web site, listed here in alphabetical order; the sentence below it provides a high-level description of what you will find on that web page.