[1][2] 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.

Rather than referring to the scale as "half-zero" or "H-zero", English-speakers have consistently pronounced it /eɪtʃ oʊ/ and have generally written it with the letters HO.

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.

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.

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.

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.

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.

Metre gauge (1,000 mm) is common in France, Germany, Switzerland, West and East Africa, parts of other countries and many tram lines.

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.

Tru-Scale made preformed wood roadbed sections, simulating ballast, that the flextrack would be fastened with tiny steel spikes.

These are representative of curves as tight as 108 feet (33 m), which in the real world would only be found on some industrial spurs and light rail systems.

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.

Today many six-axle diesels and full-length passenger cars will not run on curves less than 24 inches (610 mm) in radius.

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.

manufacturers, including Fleischmann and Märklin, developed close couplers that on straight track have the buffers almost touching, more like the prototype.

Older Australian-market train sets manufactured by Tyco, Life-Like and Bachmann used the same horn hook couplers as their American counterparts.

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: In addition to these kits, numerous manufacturers sell individual supplies for super detailing, scratch building, and kitbashing.

Toylike, ready-to-run trains using plastic molds which are well over 50 years old are still sold; at the other 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.

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.