HO scale

H0 scale (H + number zero), often also written HO scale (H + letter O), is the most popular scale model|scale of model railway in most of the world (outside the United Kingdom, where the slightly larger 00 gauge is more common). The name is derived from the fact that its 1:87 scale is approximately half that of 0 scale|0 (zero) scale, hence H0. 0 scale in turn was named following the older and larger 1 gauge|1, 2 gauge|2, and 3 gauge|3 scales. The term H0 is pronounced ("aitch-oh"), not "ho" nor "aitch-zero".

In H0 scale, 3.5 millimetres represents 1 real foot; this ratio works out to about 1:87.086. In H0, rails are usually spaced 16.5 millimeters apart which models the standard railroad gauge of 4' 8.5" or 1435 mm.

History
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.

In the 1960s, as HO scale began to overtake 0 scale in popularity, even the stalwarts of other sizes, including A. C. Gilbert Company|Gilbert (makers of American Flyer) and Lionel Corporation began manufacturing HO trains.

Currently, HO is the most popular model railroad scale in both continental Europe and North America, whereas 00 gauge is still dominant in Britain.

There are some modelers in Great Britain who use HO gauge. For them, the British 1:87 Scale Society was formed in 1994; it publishes a quarterly journal with news, views, and practical advice for modelers and collectors.

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

Controls
Modern HO trains run on realistic-looking two-rail track, which is powered by direct current (varying the voltage applied to the rails to change the velocity|speed, and polarity to change direction), or by Digital Command Control (sending digital commands to a decoder in each locomotive). Some trains, most notably by Märklin of Germany, run on alternating current, supplied by a "third rail (model railroading)|third rail" consisting of small bumps on each tie down the center of the track.

On simple, usually temporary layouts, power is supplied by a power pack consisting of a transformer and rectifier, a rheostat or potentiometer for regulating voltage supplied to the track (and thus train speed), and a switch to control train direction — a double pole, double throw slide or toggle switch wired to reverse the polarity on the rails. On permanent layouts, multiple power supplies are traditionally used, with the trackage divided into electrically isolated sections called blocks; toggle or rotary switches (sometimes relays) are used to select which power supply controlled the train in a particular block. With the advent of digital command control, block divisions are largely eliminated, as the computerized controllers can control any train anywhere on the track at any time, with minor limitations.

The Gauge
HO scale has several narrower gauges to represent narrow gauge railways in the same scale as their HO counterparts.

Curved track
Commonly manufactured curved track in HO scale may have a radius of as little as in to cm|15, which models a full-scale curved radius of ft to m|108. This is an extremely tight curve, much tighter than on real railroads, except those found in streetcar or light rail systems, or sometimes in railroads found in mountain ranges. (Standard railroads utilize curves of much greater radii.) This prefabricated snap-together track, sometimes called "snap track" — a trade name used by Atlas Model Railroad, which manufactures the track used in many HO scale toy train sets in North America — is normally used only by casual hobbyists or temporary setups. On higher-quality permanent layouts larger radii are used, frequently made of "hand laid" track whereby the modeler lays individual wooden sleepers/crossties, then installs rails on top using very small railroad spikes, followed by a layer of scale gravel ballast. Another option is so-called flex track, which is compatible with snap track. Even so, the curve radii used are still much less than those on full-size railroads. For instance, a full-size railroad curve with a radius of 800 meters (roughly 2500 feet or about one half mile) would require a radius of a bit over in to cm|345, far more than a typical "gentle" curve using hand-laid track on a high-quality layout. Curvature equivalent to that used by real railroads is simply impractical because of the limited size of the layout.

Availability of models and supplies
Because of HO scale's popularity, which is more than that of all other scales combined, a huge array of models, kits and supplies are manufactured. The annual HO scale catalog by Wm. K. Walthers, North America's largest model railroad supplier, lists more than 1,000 pages of products in that scale alone. Products run the gamut of toy train sets with ready-to-run trains and modular track, to craftsman-grade kits, supplies for building models from scratch, and even highly-detailed limited-edition locomotive models made of brass by companies based in Japan and South Korea.

Because of the wide range of products available, the scale is popular with modelers of all incomes and degrees of seriousness. For instance, ready-to-run or easy-to-assemble plastic models of freight cars may sell for less than US $5.00, while imported brass models of steam locomotives may sell for thousands of dollars or euros.

Advantages compared to other scales
HO scale's popularity lies somewhat in its middle-of-the-road status. It is large enough to accommodate a great deal of detail in finer models, more so than the smaller N scale|N and Z scale|Z scales, and can also be easily handled by children without as much fear of swallowing small parts. Models are usually less expensive than the smaller scales because of more exacting manufacturing process in N and Z, and also less expensive than S gauge|S, 0 scale|O and G scale|G scales because of the smaller amount of material; the larger audience and the resultant economy of scale also drives HO prices down. The size lends itself to elaborate track plans in a reasonable amount of room space, not as much as N but considerably more than S or 0. In short, HO scale provides the balance between the detail of larger scales and the lower space requirements of smaller scales.

HO in other hobbies and in marketing usage
In other hobbies, the term HO is often used more loosely than in railroad modeling. In slot car racing, HO does not denote a precise scale of car, but a general size of track on which the cars can range from 1:87 to approximately 1:64 scale. Small plastic model soldiers are often popularly referred to as HO size if they are close to an inch in height, though the actual scale is usually 1:76 or 1:72.

Even in model railroading, the term HO can be stretched. Some British producers have marketed railway accessories such as detail items and figures, as HO/00 in an attempt to make them attractive to modelers in either scale. Sometimes the actual scale was 00, sometimes it split the difference (about 1:82). These items may be marketed as HO, especially in the US. In addition, some manufacturers or importers tend to label any small-scale model, regardless of exact scale, as HO scale in order to increase sales to railroad modelers. The sizes of "HO" automobiles, for example, from different manufacturers, can vary surprisingly.