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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Airfix Model Railways

Airfix Model Railways

Dave McCarthy's Airfix Model Railway treasure trove is an in-depth archive of the company's plastic railway kits fr… [read review or visit site]

Gateway NMRA

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Postwar Lionel Trains Library

Postwar Lionel Trains Library

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Eric's Trains

Eric's Trains

Eric Siegel's site displays his collection of O-gauge/O-scale trains, tracks, turntables, and other accessories. A … [read review or visit site]

Tech Model Railroad Club of MIT

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Recent News: HO Scale Model Trains

Source: Google News

Model train show rolls into Portland with techie new bells and whistles (photos)
OregonLive.com, August 29th

Shron, best known as "the guy with a train in his basement," has made an HO-scale reproduction of a post-WWII Budd rail diesel passenger car. He based his model on a 3D scan of a real one. He then recorded whistles and other special sound effects, ...Read more

Weekend Fun in Fairfax County - Aug. 28-Sept. 7, 2015
Virginia Connection Newspapers, August 28th

#LEGO Model Train Show. Sunday: 12-5 p.m. Monday: 12-4 p.m. 11200 Fairfax Station Road. A two day HO scale modular train display running at the Fairfax Station Railroad Museum. They will be joined by Monty with a custom built LEGO train display...Read more

Boilermaker Special tours Linden Sunday
thepaper24-7.com, August 27th

The second building on the south side of the property houses a large HO model railroad with four operating trains, including a circus train, an animated carnival, mountain and tunnel scenery and an animated circus under construction. In keeping with...Read more

Carter Railroad Museum's Heritage Day features 'Small Trains'
WJHL, August 25th

Many people felt a sense of local pride about short line railroads, as they were often considered 'our train' by a community,” notes Geoff Stunkard, coordinator of the monthly Heritage Days program. “This was especially true here in East Tennessee...Read more

We can't stop watching this mesmerizing infinite toy train loop
Tech Insider (blog), August 24th

It's a "HO scale" train, the most popular scale for model trains. According to the National Model Railroad Association, that's a 1:86 or 1:87 ratio to the real thing. So each of these cars is 87 times smaller than an actual train car. Watching the cars...Read more

Transportation Museum hosts weekend-long Historic Spencer Shops Train Show
Salisbury Post, August 22nd

“What better place to have a train show than a place that has big, moving trains?” said Leah Cook, the museum's gift shop manager. “It brings a lot of modelers together,” she said. Cook said there are N, HO, G, S and Z scale model train vendors at the...Read more

Zone Out to This Model Train Chugging in a Never-Ending Spiral
Popular Mechanics, August 17th

Why would you want any old model train when you can have a model train that never ends? Model train enthusiast James Risner built a "bi-directional spiral" using an HO-scale train. There's nothing particularly noteworthy about the train itself, other...Read more

You Can Watch Forever But the End of This Infinite Model Train Will Never Arrive
Gizmodo, August 16th

Powered by what appears to be seven HO-scale locomotives, Risner's creation also works in reverse. But based on our scientific calculations, which are in turn based on the science we learned from the original Superman movie, leaving this setup running...Read more