HO scale model trains are 1/87 the size of real trains and 1/2 the size of O scale model trains (HO stands for "half O"). As with O scale trains, the letter "O" in HO is a misnomer since the designation was initially conceived as a zero to identify trains that were smaller in scale than 1. But the use of the letter O rather than the numeral 0 crept into everyday speech among model-train buffs and remains the standard way to refer to HO scale today.

Though first invented in the 1930s, HO scale model trains didn’t become popular in the U.S. and Europe until the 1950s when the scale was heavily promoted by Märklin, Lionel, Athearn, and others. HO never quite gained acceptance in England, though, where larger 00 scale trains still predominate.

HO has two important benefits over trains of larger sizes. First, an HO scale layout of reasonably ambitious detail and complexity can fit on a table rather than requiring an entire room, as is the case with larger scales. Second, HO trains are less expensive to manufacture than OO, O, and other large-scale trains, which makes them more affordable to collectors and enthusiasts alike.

Most HO scale trains run on two-rail tracks using direct current (DC). Adjusting the voltage delivered to the tracks can increase or decrease the speed of the trains; reversing the polarity causes the trains to roll in reverse. Some manufacturers, notably Märklin, produce HO scale trains that operate on alternating current (AC), which is delivered to the train via a conductive third rail.

The gauge used by the majority of HO scale model trains is a standard HO gauge track that’s roughly proportional to the size of the trains running on top of it. But as with the OO scale trains in England that run on HO gauge tracks, HO scale trains are also made to run on a variety of different gauge tracks, which themselves are scaled-down versions of historical railroad track sizes.

Of these variants, the most common is called HOn3. This code denotes an HO scale train running on a track similar to the 3-foot, narrow gauge tracks used by Rocky Mountain railroad operators in the late 19th century.

One of the most enjoyable aspects of HO for collectors is the opportunity to join a local module club. Members of such clubs build modules of predetermined size; each module has a finite length of track. When members get together at model train exhibitions or in private homes, the modules are linked up and trains are run on the resulting massive layout, which often feels like more than just the sum of its parts.


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