Railroad tank cars carry an enclosed tank on their wooden flat car, steel frame, or, later, just their trucks. These cars are used to transport any kind of liquid or gaseous material. The vast majority simply hold their content, while others are able to maintain their content at a certain temperature, and yet others can keep their content under pressure.

Tank cars were generally not owned by railroads themselves, but rather by either their manufacturer or by other companies, which most of the time leased them out to another company. You can identify these by the "X" in their reporting marks, meaning that the car's owner is not a common carrier.
In 1865 crude oil was discovered in Pennsylvania, which necessitated the first "tank car", nothing but a wooden flat car with some tubs on it. A few years later metal tanks were developed to cut down on the leaking. Around 1900 the wooden cars were no longer able to carry the weight and so tank cars were made out of steel frames. Tanks were riveted together, but as content was starting to be put under pressure, this caused serious problems. By the start of WWII welded tanks quickly became the standard. In 1954 the modern "frameless" tank car came out, where the tank itself was part of the frame of the car. By 1960 tank cars also became "domeless". Tank cars nowadays use Type F couplers, which are designed to stay coupled even during a derailment, to prevent cars from coming loose in an accident.
Below are links to the various reports that show all of the S-scale reefers 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:
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