HO is the most popular railroad scale model in both continental Europe and North America. The HO name is derived from the fact that its 1:87 scale is approximately half that of O scale which was the smallest of the series of older and larger 0, 1, 2 and 3 scales introduced by Marklin around 1900.

The HO train developed and first appeared in United Kingdom in response to great depression created by economic pressures. It became popular in United States in the late 1950s and began to challenge the market dominance of 0 gauge. In 1960s HO began to outshine 0 scale in popularity that even the maker of American Flyer and Lionel Corporation began producing HO trains.