Trains come in a variety of sizes, generally measured by scale (e.g., American Flyer’s S scale) or gauge (e.g., Lionel O gauge). They approximately measure the same thing, but are different. The scale represents how the trains are reduced in size relative to the full-size objects they represent. O scale, for examples, means that objects are reduced by a factor of 43.5. Thus a 12 inch model car represents 12 times 43.5 inch or 522 inch full sized car (e.g., 43.5 feet). The gauge represents how far apart the rails are. Standard gauge (full-sized track) is 4 feet 8 ½ inches apart. O gauge is defined as track that is 1 ¼ inches apart. Note that O gauge is actually a reduction in size of 45.2 (56.5 inches divided by 1¼ inches), close to O scale, but not exactly the same.

At Riderwood, we have examples of the 5 common sizes used by railroad hobbyists. From smallest to largest, these are:

1. Z scale: This is the smallest scale with a reduction factor of 220:1. It was introduced by Marklin, a German toy manufacturer in 1972. Due to its size (a 50 foot box car would be less than 3 inches long) it is hard to easily model details or even to see those details and it is not particularly popular. The main advantage is you can create a huge railroad empire on a 4×8 foot plywood board.
2. N scale: This scale is a reduction factor of 148:1. It is quite popular in the U.S. with those who want a railroad and only have a limited amount of space.  They are often used as part of a 2 foot wide shelf layout. Built along one wall of a room, the 2 foot width and the 148:1 size reduction allow for a reasonable number of tracks on which to switch trains onto with sufficient space for realistic scenery and train operations.
3. HO scale: Today, HO scale, at a reduction of 87:1, is the most popular scale in which to model railroads. Most cars are from 6 to 8 inches long and engines run about 9 or 10 inches. You can get an interesting layout on a 4×8 foot board, and within a large room, you can model complex track designs with sufficient detail to provide for realistic modeling of cars, engines, and buildings. The main Riderwood layout is HO scale. The first truly popular scale in the U.S. was O gauge (described below) from around 1900. HO developed in the 1920s as “Half O”, since its scale was half of O scale’s 43.5:1 or a reduction of 87:1.
4. S scale: S scale, or a ratio of 64:1 is approximately midway between HO scale at 87:1 and O gauge at 43.5:1. It developed in the U.S. in the late 1930s. After World War II until the early 1960s, American Flyer, produced by A. C. Gilbert was built in S scale, and Lionel, its main competitor, was built in O gauge. If you had a railroad in the 1950s as a kid, you probably had one of these sets.
5. O gauge: O gauge was the dominant size sold in the U.S. during the 1950s. Lionel outsold American Flyer by a factor of 2 to 1. The goal here was durability for rough handling by pre-teen boys and not necessarily realism in the cars and engines. Note that Lionel trains are called O gauge, not O scale. That was because the track width was a true 1¼  inches (e.g., a scale of 43.5:1), but cars were a bit shorter than the O scale demanded. Lionel (and others) also produced an O27 gauge. These were built to a scale of 64:1, but used the same rail width as O gauge (hence the cars are smaller than true O scale). The “27” referred to the diameter of a circle required to turn the train.

Lionel and American Flyer trains run on AC electricity, whereas Z, N, and HO are DC powered.  Note that the Lionel tracks are 3 rails. The center rail is one terminal of the AC circuit while the outside rails are the other terminal. True O scale modelers run DC-powered two rail trains, much like the other DC powered scales.

Other scales: Railroads come in other less popular sizes. G scale Garden railroads (G for German Gross or big) are often used on outdoor layouts and are built to a scale of 22.5:1. These are approximately twice as large as O scale. There is also the very small T gauge at 450:1. More experienced modelers also look at more realistic scales such as On30 narrow gauge which uses O scale trains on HO gauge track.