A while back a friend of mine was reminiscing about his childhood model trains. I asked what scale they were and he replied "Standard Gauge". Now I knew the gauge of his toy trains wasn't 4 ft. 8 1/2 inch (real life standard gauge) , so I did some research and found that toy trains in "Standard Gauge" were made by Lionel in the 1930s, and that Lionel stopped producing them in 1940. I asked him if his trains had been made in the 30s, knowing that these would have been old by the time when we were born. He replied, "No, they were Marx trains from the 60s". I told him in that case they were O scale/gauge.
Ironically, I had forgotten about "Standard Gauge" when initially responding to a question in our forum. In the forum "dushyantdhami" asked about 2.25 inch gauge trains. I couldn't recall reading about any 2.25 inch gauge model railroad trains. My article on scales and gauges cover the ones I know to be currently manufactured and marketed somewhere in the world today. I referred his toy train's to the most comprehensive, if no easy to use, list of scales and gauges that I know of; Christopher Coleman's Model Railroad Scale List. This is a great resource, but he uses the words "Standard Gauge" repeatedly in giving the gauge in each scale for modeling 4 ft. 8 1/2 inch gauge trains.
Later that evening I was reading "Classic Toy Trains" by Gerry and Janet Souter and came across a photo of two Lionel locomotives from the 1930s side by side. One was O scale, and the other was Standard Gauge. The larger Standard Gauge model sat on three-rail tubular tracks just like the O scale model. But though it was obvious that the Standard Gauge tracks were wider, there was no reference to the actual gauge of the tracks. A search of the web yielded this Standard Gauge page. According to this page, and other's I've since found, Standard Gauge was 2.125 inches. However, there appear to have been some manufacturers who made comparable trains at 2.25 inch gauge. These were called "Wide Gauge", as Lionel had copyrighted the name "Standard Gauge".