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 mm (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 previously the smallest of the series of older and larger 0, 1, 2 and 3 gauges introduced by Märklin around 1900. In most English-speaking markets it is pronounced “aitch-oh” and written with the letters HO today, but in other markets remains written with the letter H and number 0 (zero), so in German it is pronounced as “hah-null”.
Most modern HO trains run on two-rail track powered by direct current (varying the voltage applied to the rails to change the speed, and polarity to change direction), or by Digital Command Control (DCC) (sending digital commands to a decoder in each locomotive to set the speed, change direction and activate sounds and lights while power comes from the track which is always energised). Other trains, such as those manufactured by Märklin, run on alternating current and use a “third rail” of small protruding studs between the running rails.
On simple, usually temporary layouts, power is supplied by a power pack consisting of a transformer and rectifier (DC), a rheostat for regulating power to the track (and thus train speed), and reversing to control model direction. 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 controls 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.
HO scale steam locomotives at the N&W RR museum in Crewe, Virginia.
For Europe it is defined in the Normen Europäischer Modellbahnen standard NEM 010 published by MOROP as exactly 1:87. For North America the National Model Railroad Association (NMRA) standard S-1.2 defines HO scale 3.5 mm (0.1378 in) as representing 1 real foot (304.8 mm)—a ratio of 1:87.0857142, usually rounded to 1:87.1. The precise definition of HO or H0 scale varies slightly by country and manufacturer.
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 Britain. There are some modellers in Great Britain who use HO scale, and the British 1:87 Scale Society was formed in 1994.
Advertising gift of a Mercedes bus in HO
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 one inch (25 mm) high, 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/OO” in an attempt to make them attractive to modelers in either scale. Sometimes the actual scale was OO, 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 greatly.
The “gauge” of a rail system is the distance between the inside edges of the railheads. It is distinct from the concept of “scale”, though the terms are often used interchangeably in rail modelling. “Scale” describes the size of a modeled object relative to its prototype. Prototype rail systems use a variety of track gauges, so several different gauges can be modeled at the same scale.
The gauges used in HO scale are a selection of standard and narrow gauges. The standards for these gauges are defined by the NMRA (in North America) and the NEM (in Continental Europe). While the standards are in practice interchangeable, there are minor differences.