- Number 5 gauge - 120 mm - 4 ft 5/8 in - 1:8 - Also known as V Gauge.
- Number 4 gauge - 75 mm - 3 in - 1:11 or 1:20 - Also known as IV or 3 gauge. Measurement is sometimes also quoted at 2 15/16 in.
- Number 3 gauge - 67 mm - 2 5/8 in - 1:16 or 1:22 or 1:23 - also known as III, II, IIa gauges.
- Number 2 gauge - 54 mm - 2 1/8 in - 1:22.5 or 1:27 or 1:28 - also known as II gauge.
- Number 1 gauge - 45 mm - 1 7/8 in - 1:32 or 1:30 - Also known as I gauge. Used by modern G scale.
- Number 0 gauge - 35 mm - 1 3/8 in - 1:48 or 1:43 or 1:45 or 1:64 - Introduced later, around 1900. This is modern O gauge.
Märklin measured the gauge as the distance between the center of the two outer rails, rather than the distance between the outer rails themselves. Lionel's Standard gauge is allegedly the result of Lionel's misreading these standards, as are the variances in O gauge between the United States and Europe.
Most of these standards never really caught on, due to their large size, which made them impractical to use indoors, as well as the high price of manufacturing. Wide gauge trains, which are close in size to 2 gauge, are produced in limited quantities today, as are 1 gauge and O gauge trains. Of these, O gauge is the most popular.
The modern standards for toy trains also include S gauge, HO scale, and N scale, in descending order of size. HO and N scale are the most popular model railway standards of today; inexpensive sets sold in toy stores and catalogs are less realistic than those sold to hobbyists. O gauge arguably remains the most popular toy train standard.
Although the words "scale" and "gauge" are often used interchangeably, toy train manufacturers have only recently concerned themselves with accurate scale. The terms "O scale" and "S scale" tend to imply serious scale modeling, while the terms "O gauge" and "S gauge" tend to imply toy trains manufactured by the likes of Lionel and American Flyer. While S gauge is fairly consistent at 1:64 scale, O gauge trains represent a variety of sizes. O gauge track happens to be 1/45 the size of real-world standard gauge track, so manufacturers in Continental Europe have traditionally used 1:45 for O gauge trains. British manufacturers rounded this up to 1:43, which is seven millimeters to the foot. U.S. manufacturers rounded it down to 1:48, which is a quarter-inch to the foot. However, most engaged in a practice of selective compression in order to make the trains fit in a smaller space, causing the actual scale to vary, and numerous manufacturers produced 1:64 scale trains—the proper size for S gauge—in O gauge, especially for cost-conscious lines.
Some of the earliest O gauge trains made of tinplate weren't scale at all, made to unrealistic, whimsical proportions similar in length to modern HO scale, but anywhere from one and a half to two times as wide and tall.
Some adult fans of toy trains operate their trains, while others only collect. Some toy train layouts are accessorized with scale models in an attempt to be as realistic as possible, while others are accessorized with toy buildings, cars, and figures. Some hobbyists will only buy accessories that were manufactured by the same company who made their trains. This practice is most common among fans of Marx and Lionel.