As with prototype trains, gauge is the measure of the distance between the rails. Standard Gauge has two different meanings with model trains:
1. On prototype railroads in most of the world, the majority of tracks are laid in a common, or "standard" gauge. This gauge of 4 feet 8.5 inches began evolving as the standard as early as the 1840s. By sharing a common gauge, connecting railroad companies can interchange, or exchange cars.
Most model train scales are also built to confirm to a common gauge that at least approximates standard gauge. Most O scale trains are built to an even 5 foot gauge. G gauge trains share a common gauge but are actually sized differently to make that gauge appear either standard or narrow.
Trains running on tracks wider or more narrow than 56.5" are said to be broad or narrow gauge respectively. Narrow gauge trains are far more common. These too are modeled in almost every scale. For model trains, a narrow gauge is indicated by the letter "n" after the scale, followed by a number which indicates the gauge. "On3" would be 3' narrow gauge in O scale for example.
2. With model trains, standard gauge can also refer to a size of model trains produced by Joshua Lionel Cowen (Lionel) from 1906 to 1939. These trains were slightly larger than the O scale trains which superseded them with an actual track gauge of 2 1/8". Lionel began producing O scale trains, with a gauge of 1 1/4", in 1915.
Today, these original standard gauge trains are highly collectible and can bring big money in good condition. Reproductions have also been made by several companies.