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 apart for modelling 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 pronounced it 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 . In Japan, many models are produced using 1:80 scale (16.5 mm track is still used).

History

thumb|HO-scale model of a [[CSX Transportation|CSX locomotive]]

thumb|First model railroad layouts in today's H0 gauge, 1926

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 was designed to represent prototypical standard-gauge track of 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, while 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 precise definition of HO scale thus varies slightly by country and manufacturer.

thumb|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 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 both scales. Sometimes the actual scale is OO, and sometimes the difference is split (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, can vary greatly between different manufacturers.

Power and control

thumb|East Texas Model Railroad Club HO-scale layout's yard switch controls

Model locomotives are fitted with small motors that are wired to pick up power from the rails. As with other scales, HO trains can be controlled in either analog or digital fashions. With analog control, two-rail track is powered by direct current (varying the voltage applied to the rails to control speed, and polarity to control direction). With digital control, such as Digital Command Control (DCC) or proprietary systems such as the one developed by Märklin, digital commands are encoded at the controller and received by any decoders receiving power from the track. Digital control allows independent control of each locomotive's speed and direction as well as functions not easily achieved with analog control such as reactive sound and lighting effects, integration of auxiliary decoders and automation.

The basic power and control system consists of a power pack of a transformer and rectifier (DC), a rheostat. On large model layouts, the power system may consist of several signal boosters, control interfaces, switch panels and more. Trackage may be divided into electrically isolated sections called blocks and toggle or rotary switches (sometimes relays) are used to select which tracks are energized. Blocking trackage also allows the detection of locomotives within the block through the measurement of current draw.

Track

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.

{| class="wikitable sortable"

|+ Track gauges used in HO/H0-scale modelling

!data-sort-type="number"| Track gauge !!class="unsortable"| NEM HOn2 uses .

|-

| || H0f || ||data-sort-value="600"| – gauge || 90 px || Feldbahn, Trench- or Light railway, H0f can uses commercially available Z-scale track. Defined in NEM 010 for modelling gauge including 2 ft and 600 mm gauge systems.

The reference will then add itself to the footnote section.

</nowiki> -->

  • Layout Tours A-L and Layout Tours M-Z
  • Tony Cook's HO-Scale Trains Resource Includes separate web sites for many classic and contemporary HO-scale model train product lines of the past; online catalog resources; plus links to current manufacturers.
  • British 1:87 Scale Society with the online magazine Satellite 1:87