Standard gauge track has a distance of 2.26" between running rails.  There probably never was an official Standard scale because the peak popularity of this gauge predated widespread interest in scale model railroading.  It was the premier toy train gauge from World War I through the early 1930s.

 

Lionel coined the term Standard gauge and claimed the exclusive rights to its use.  Both American Flyer and Ives also manufactured trains with this track configuration, but referred to them as Wide gauge.  By the turn or the 1940s, Sandard gauge toy trains had all but vanished from the scene.

 

A few independent manufacturers began producing new and original Standard gauge items for collectors as early as the 1970s, but it was not until the classic Lionel trains were reproduced in the middle of 1980s that significant interest in the gauge revived.