HO Scale Model Trains

HO scale is the most popular scale of model railway in most of the world (outside the United Kingdom, where the slightly larger 00 gauge is most common). The name is derived from the fact that its 1:87 scale is approximately half that of O scale, hence H0 or 'half zero'.

In HO scale, 3.5 millimetres represents 1 real foot; this ratio works out to about 1:87.086. In HO, rails are usually spaced 16.5 millimeters apart which models the standard railroad gauge of 4' 8.5" or 1435mm. (...)
HO scale trains first appeared in the United Kingdom in the 1930s, originally as an alternative to 00 gauge, but could not make commercial headway against the established 00 gauge. However, it became very 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 0 scale, its smaller size allows modelers to fit more details and more scale miles into a comparable area.

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