O Scale

At the turn of the last century, Lionel was the first to produce electric toy trains in America in what they called “Standard Gauge,” where the rails were 2.125″ apart. After the first World War, the smaller O scale(1:48) became the popular size for toy train manufacturers in America. Many toy train sets run on three-rail track, which allows for complex track layouts to be built without complicated electrical wiring. Lionel continues to make O scale trains and accessories, along with a number of other manufacturers like Atlas, MTH, and Williams.

From: Railroad Model Craftsmen Magazine December 25, 2016
http://rrmodelcraftsman.com/model-train-scale-gauge/