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HO is the most popular scale of model railway in the United States. The name is sometimes thought to be derived from Half-O, because its 1:87 scale is approximately half that of O scale.

HO scale trains first appeared in the United Kingdom in the 1930s, originally as an alternative OO scale, but became much more popular in the United States, where it took off in the late 1950s after interest in model railroads as toys began to decline and more emphasis began to be placed on realism in response to hobbyist demand. While HO scale is by nature more delicate than O scale, its smaller size allows modelers to fit more details and more scale miles into a comparable area.

In the 1960s, as HO scale began to overtake O scale in popularity, the stalwarts of other sizes, including Gilbert (makers of American Flyer) and Lionel began manufacturing HO trains. HO locomotives and rolling stock (cars) are available today from a large number of manufacturers.