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|Scale||3.5 mm to 1 foot|
|Model gauge||16.5 mm (0.65 in)|
|Prototype gauge||Standard gauge|
|Scale||3.5 mm to 1 foot|
|Model gauge||16.5 mm (0.65 in)|
|Prototype gauge||Standard gauge|
According to the NMRA standard S-1.2 predominantly used in North America, in HO scale, 3.5 mm (0.1378 in) represents 1 real foot (304.8 mm); this ratio works out to 1:87.0857142, usually rounded to 1:87.1. According to the MOROP standard NEM 010 predominantly used in Europe, the scale is exactly 1:87. In HO, rails are usually spaced 16.5 mm (0.64961 in) apart which models the standard railroad gauge of 1,435 mm (4 ft 8 1⁄2 in).
The name HO 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 Märklin around 1900. In most English-speaking markets it is pronounced "aitch-oh" and written with the letters HO today, but in German it is pronounced "hah-null", and still written with the letter H and numeral 0.
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 was designed to represent prototypical standard gauge track, 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 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 miles into a comparable area.
In the 1950s HO began to challenge the market dominance of 0 gauge and, in the 1960s, as it began to overtake 0 scale in popularity, even the stalwarts of other sizes, including 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 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. For them, the British 1:87 Scale Society was formed in 1994; it publishes a quarterly journal with news, views, and practical advice for modellers and collectors. A magazine, Continental Modeller, focuses on the railways of other countries, including America and Europe, and has extensive coverage of HO scale layouts.
Modern HO trains run on two-rail track, which is 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). Some trains, most notably by Märklin of Germany, run on alternating current, supplied by a "third rail" consisting of small bumps on each tie down the centre 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 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, with minor limitations.
"Gauge" and "scale" are used interchangeably in the hobby; however, this is not correct terminology. "Scale" refers to the size of the object in question, whereas "gauge" only means the width between the insides of the rails. As noted below, HO "scale" has numerous "gauges", all at 1:87 scale.
HO scale has several gauges representing both standard and narrow gauges in roughly 1:87 scale. Standards are defined by the NMRA (in North America) and the NEM (in Continental Europe). While the standards are in practice interchangeable, they are not strictly identical.
|16.5 mm (0.65 in)||HO (NMRA) and H0 (NEM)||Standard gauge||16.5 mm (0.65 in) track is also used for British OO gauge and Sn3½ scale (to represent narrow gauge).|
|12 mm (0.472 in)||HOn3½ (NMRA) and H0m (NEM)||Metre gauge and 3 ft 6 in (1,067 mm) gauge||Metre gauge is used in west and east Africa, parts of other countries and many tram lines. 3 ft 6 in (1,067 mm) gauge is used in southern Africa, Australia (Queensland, Tasmania, and Western Australia), New Zealand, and also non-Shinkansen JR lines in Japan. H0m and HOn3½ use commercially available TT scale track.|
|10.5 mm (0.413 in)||HOn3 (NMRA)||3 ft (914 mm) gauge||3 ft (914 mm) gauge once common to American mining railroads and shortlines, particularly in the Western States|
|9 mm (0.354 in)||HOn30 (NMRA) and H0e (NEM)||2 ft 6 in (762 mm) gauge||Typically used for lines in 2 ft (610 mm)-2 ft 6 in (762 mm) gauge. Uses commercially available N scale track.|
|6.5 mm (0.256 in)||HOz (NMRA) and H0i (NEM)||15 in (381 mm) gauge||Uses commercially available Z scale track.|
The earliest "pre-gauged" track available in the 1940s had steel rails clipped to a fiber tie base. This was called flexible track as it could be "flexed" around any curve in a continuous fashion. The sections were sold as three-foot lengths, and the rail ends were connected with a sheet metal track connector that was soldered to the base of the rail.
As brass became more readily available, the steel rail was phased out, along with its corrosion problems. Brass flex-track continued to be available long after sectional track was introduced, as the three-foot lengths of rail reduced the number of joints. The biggest disadvantage of flex-track was that it had to be fastened to a roadbed.
In the late 1940s, Tru-Scale made milled wood roadbed sections, simulating ballast, tie plates and milled ties with a gauged, grooved slot with simulated tie plates. Bulk HO code 100 rail was spiked in place with HO spikes. This was available in straight lengths and curves, from 18-inch radius to 36-inch radius. It was up to the user to stain the wood for the tie colors prior to laying the brass track, and then adding scale ballast between the ties.
Tru-Scale made preformed wood roadbed sections, simulating ballast, that the flextrack would be fastened with tiny steel spikes. These spikes were shaped much like real railroad spikes, and were fitted through holes pre-drilled in the fiber flextrack ties base. An improvement was made when "sectional track" became available in a variety of standardized lengths, such as the ubiquitous 9 in (228.6 mm) straight and curved tracks of 15 in (381.0 mm), 18 in (457.2 mm), and 22 in (558.8 mm) radii. These are representative of curves as tight as 108 feet (32.9 m), which in the real world would only be found on some industrial spurs and light rail systems.
Sectional track was an improvement in setting up track on a living room floor because the rail was attached to a rigid plastic tie base, and could withstand rough handling from children and pets without suffering much damage. With flex track, which can be bent to any desired shape (within reason), it became possible to create railroads with broader curves, and with them more accurate models. Individual rails are available for those that wish to spike their own rails to ties. Individual ties can be glued to a sound base, or pre-formed tie and ballast sections milled from wood can be used for a more durable, if somewhat artificially uniform, look is preferred.
There are a variety of preassembled track sections made by Märklin using their unique three-rail system. This trackwork is a little bulkier looking than true to scale, but it is considered quite trouble-free, and is preferred by many that are interested in reducing much of the operational problems that come with HO scale railroading. As with other preformed track, it is also available in several radius configurations. Generally speaking very sharp radius curves are only suitable for single unit operation, such as trolley cars, or for short-coupled cars and locos such as found around industrial works. Longer wheelbase trucks (bogies) and longer car and loco overhangs require the use of broader radius curves. Today it is common to purchase six-axle diesels and full-length passenger cars which will not run properly on curves less than 24 in (610 mm) in radius.
HO scale track was originally manufactured with steel rails on fiber ties, then brass rail on fiber ties, then brass rail on plastic tie. Over time, track made of nickel silver (an alloy of nickel and brass) became more common due to its superior resistance to corrosion. Today, almost all HO scale track is of nickel silver, although Bachmann, Life-Like and Model Power continue to manufacture steel track.
In America, Atlas gained an early lead in track manufacturing, and their sectional, flex, and turnout track dominates the US market. In the UK, Peco's line of flex track and "Electrofrog" (powered frog) and "Insulfrog" (insulated frog) turnouts are more common. Atlas, Bachmann, and Life-Like all manufacture inexpensive, snap-together track with integral roadbed. Kato also manufactures a full line of "HO Unitrack", however it has not yet caught on as their N scale Unitrack has.
Rail height is measured in thousandths of an inch; "code 83" track has a rail which is .083" high. As HO's commonly available rail sizes, especially the popular "code 100", are somewhat large (representative of extremely heavily trafficked lines), many modelers opt for hand-laid finescale track with individually laid wooden sleepers and crossties and rails secured by very small railroad spikes.
In Australia, many club-owned layouts employ code 100 track so that club members can also run OO-scale models and older rolling stock with coarse (deep) wheel flanges.
Because of the scale's popularity, 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. Models are generally available in three varieties:
Quality varies extremely. Toylike, ready-to-run trains using plastic molds which are well over 50 years old are still sold; on the other side are highly detailed limited-edition locomotive models made of brass by companies based in Japan and South Korea. A popular locomotive such as the F7/F9 may be available in thirty different versions with prices ranging from twenty to several thousand dollars or euros.
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 and 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, O and 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.
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 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.
Currently active significant manufacturers and marketers of HO railroad equipment as of 2009, include, but are not limited to:
Significant historical manufacturers and marketers of HO equipment which are no longer active in HO, include
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