Among the most common sizes are "O Gauge", "S Gauge", and "HO Gauge". Here is a table which provides the most common understanding of the ratios which are associated with each gauge.

One way to visualize the meaning of the ratios is to understand that an O Gauge model train would require forty eight box cars piled one on top of the other to equal the height of a real train, a S Gauge train would require sixty four boxcars piled one on top of the other to equal the height of a real train, and an HO Gauge train would require 87 boxcars piled one on top of the other to equal the height of a real train.

It follows that the smaller the ratio, the larger the model train car. An O gauge model train car is larger than a S Gauge model train car, and a S Gauge model train car is larger than an HO Gauge model train car.

The ratios can also be expressed as rough percentages of the relative sizes of model train cars and real train cars, as per the table. Thus, an O Gauge boxcar is about 2.1 percent of the size of a real boxcar, a S Gauge boxcar is about 1.5 percent of the size of a real boxcar, and an HO Gauge boxcar is about 1.2 percent of the size of a real boxcar.

Although the standards noted above are normative, various manufacturers make model trains labeled as one or another of the Gauges but which are sized differently from those made by other manufacturers. This website is concerned primarily with S Gauge model trains so we will comment on the variations which one is likely to find among model trains labeled by their manufacturers as "S Gauge."

In the United States, the first notable maker of S Gauge model trains was the A. C. Gilbert Company offering models under the "American Flyer" brand name. Since the late 1970s American Flyer trains have been manufactured in S Gauge by Lionel. The shells of typical American Flyer boxcars measure about 7 3/4 inches long by about 1 7/8 inches high. Note, the shell measurement does not include wheels and couplers.

Among other S Gauge model train manufacturers which produce boxcars of a very similar size to the original Gilbert standared are American Models, S-Helper, and MTH.

The shells of S Gauge boxcars manufactured by K-Line measure about 8 3/8 inches long by about 2 3/16 inches high, just about one half inch longer and one quarter inch higher than the S Gauge standard set by Gilbert.

The shells of S Gauge boxcars manufactured by S Scale America (Des Plaines Hobbies) and Pacific Rail measure about 9 3/4 inches long by about 2 1/2 inches high, just about two inches longer and 3/4 of an inch higher than the S Gauge standard set by Gilbert.

The following table may be helpful to S Gauge enthusiasts:

Typically, but not always, all trains manufactured to S Gauge standards will couple and operate together on the same two rail track even though the shells are of different sizes. A major exception to this generalization are S Gauge toy trains which employ different styles of couplers which some modelers prefer to use to maximize conformity of appearance to real trains. The most common examples of such couplers are the Kadee couplers frequently used by "scale" operators of toy trains.