HO scale

HO or H0 is a rail transport modelling scale using a 1:87 scale (3.5 mm to 1 foot). It is the most popular scale of model railway in the world. The rails are spaced 16.5 millimetres (0.650 in) apart for modelling 1,435 mm (4 ft 8+1⁄2 in) standard gauge tracks and trains in HO.

The name HO comes from 1:87 scale being half that of O scale, which was originally the smallest of the series of older and larger 0, 1, 2 and 3 gauges introduced by Märklin around 1900. Rather than referring to the scale as "half-zero" or "H-zero", English-speakers have consistently ^{[citation needed]} pronounced it /eɪtʃ oʊ/ and have generally written it with the letters HO. In other languages it also remains written with the letter H and number 0 (zero); in German it is thus pronounced as [ha: 'nʊl]. In Japan, many models are produced using 1:80 scale (16.5 mm track is still used).

After the First World War there were several attempts to introduce a model railway about half the size of 0 scale that would be more suitable for smaller home layouts and cheaper to manufacture. H0 was created to meet these aims. For this new scale, a track width of 16.5 mm (0.65 in) was designed to represent prototypical standard-gauge track of 1,435 mm (4 ft 8.5 in) width, and a model scale of 1:87 was chosen. By as early as 1922 the firm Bing in Nuremberg, Germany, had been marketing a "tabletop railway" for several years. This came on a raised, quasi-ballasted track with a gauge of 16.5 mm, which was described at that time either as 00 or H0. The trains initially had a clockwork drive, but from 1924 were driven electrically. Accessory manufacturers, such as Kibri, marketed buildings in the corresponding scale.

At the 1935 Leipzig Spring Fair, an electric tabletop railway, Trix Express, was displayed to a gauge described as "half nought gauge", which was then abbreviated as gauge 00 ("nought-nought"). Märklin, another German firm, followed suit with its 00 gauge railway for the 1935 Leipzig Autumn Fair. The Märklin 00 gauge track that appeared more than ten years after Bing's tabletop railway had a very similar appearance to the previous Bing track. On the Märklin version, however, the rails were fixed to the tin 'ballast' as in the prototype, whilst the Bing tracks were simply stamped into the ballast, so that track and ballast were made of a single sheet of metal.

HO scale trains elsewhere were developed in response to the economic pressures of the Great Depression. The trains first appeared in the United Kingdom, 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 track distance into a comparable area.

In the 1950s HO began to challenge the market dominance of O gauge and, in the 1960s, as it began to overtake O scale in popularity, even the stalwarts of other sizes, including Marx and Lionel Corporation, began manufacturing HO trains.

Today, HO locomotives, rolling stock (cars or carriages), buildings, and scenery are available from a large number of manufacturers in a variety of price brackets.

HO is the most popular model railroad scale in both continental Europe and North America, whereas OO scale (4 mm:foot or 1:76.2 with 16.5 mm track) is still dominant in the United Kingdom. There are some modellers in the United Kingdom who model in HO scale and the British 1:87 Scale Society was formed in 1994.