Since the mid-1800's, the model train has allowed every day folks the opportunity to run their own railroad system. The first model train sets were crude representations of the real thing, and when electric train sets hit the scene, hobbyists could run their own small tracks and watch their models slide along the rails. Model train sets have evolved over the years, and scale models come in all different sizes now. Hobbyists can create their own scenery, organize their own train yards, and link their own cars to create a train set as elaborate or as simple as they like.
Model train sets come in different scales, or sizes, and are modeled after actual train cars. The likeness is much more accurate now than in decades past. The most common scales G scale, HO scale, and N scale. These represent different ratios; for example, in the HO scale, 3.5 mm equals approximately one real foot. The model train runs on metal rails that are charged with a direct current to propel the engine. A conductor may vary the voltage to control the speed, or the polarity to reverse direction.
Model train sets became popular in the United States in the 1950's when hobbyists began to value realism over the inaccuracy of toy trains. HO scale became the most popular scale because it allowed for a realistic look but also let the hobbyist create a set for the train in a more confined space. HO trains are the most readily available in the U.S., and accessories such as landscape and buildings are cheaper and easier to come by in this scale size.
Hobbyists have taken to building their model train layouts to fit a particular time period or location. For example, a layout may be built around an actual running railroad such as the New Haven line in Connecticut and New York, or the Santa Fe line in the southwest United States. More intense hobbyists will strive for realism, and will therefore attempt to make sure there are no anomalous aspects to their layouts -- for example, a modern automobile driving down the street of their 1950's layout. There are several publications on newsstands to help hobbyists learn what model train is best for them and what layout might suit their needs.