Trackage
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 continues 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.
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 Marklin 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 (609.6 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 was up until quite recently manufacturing 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/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.
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