HO Scale Model Railroad, which uses a scale of 1:87 (3.5 mm to 1 foot). The rails are spaced 16.5 mm (0.650 in) apart for modeling 1,435 mm (4 ft. 8 1⁄2 in) standard gauge tracks and trains in HO. The HO scale is the most popular in railway modeling worldwide. The name HO comes from the 1:87 scale being half that of the O scale, which used to be the smallest of the series of older and larger 0, 1, 2, and 3 gauges, which were introduced around 1900 by Märklin. In English speaking markets, it is written as HO, but in other markets, the letters H and number 0 (zero) are used.
The term HO can be stretched in model railroading. Some manufacturers in the UK have marketed railway items such as figures and detail items as “HO/OO” to appeal to modelers in both scales. The actual scale sometimes is OO, and the difference is negligible (about 1:82). These items may therefore be marketed as HO, especially in the US. Some producers also tend to label any small-scale model as an HO scale regardless of the scale to improve their sales to hobbyists and modelers. For example, there may be a great variation in the sizes of HO automobiles from different manufacturers.
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 modeling. “Scale” describes the size of a modeled object relative to its prototype. Prototype rail systems use various track gauges to model several different gauges on the same scale.
The gauges used in the HO scale are a selection of standard and narrow gauges. The standards 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.
Due to the huge popularity of the HO scale, a wide array of models, supplies, and kits are produced. The annual HO scale catalog by Wm. K. Walthers, North America’s largest model railroad supplier, lists more than 1,000 pages of products on that scale alone. Models are generally available in three varieties:
- Ready-to-run models are fully ready for use right out of the box. Generally, this means couplers, trucks (bogies), and other integral parts are installed at the factory, although some super detailing parts may still need to be attached.
- Shake-the-box kits are simple, easy-to-assemble kits; a freight car might include a one-piece body, a chassis, trucks, couplers, and a weight, while a structure kit might include walls, windows, doors, and glazing. The name derives from the joke that no skill was required – shake the box, and the kit falls together. A common synonym is a screwdriver kit, as many can be assembled with a screwdriver and tweezers.
- Craftsman kits require a much higher level of skill to assemble and can include several hundred parts.
Several manufacturers also market individual supplies and these kits for super detailing, kitbashing, and scratch building.