In 1906 Lionel started producing electric trains in what they called "Standard Gauge" - this after their self proclaimed slogan "Lionel - Standard of the World". They produced these trains until 1937.
Other manufacturers of the time; IVES, Dorfan and American Flyer also produced trains of the same gauge, but they called it "wide gauge".
These were large tin trains (steamers were mostly die-cast) with many bright colors and brass or nickel trim. I've accumulated a number of Lionel's Standard gauge trains, some original and some I restored (Such as the grey passenger set show in the photos).
My "collection" started with a flea market purchase of a derelict Lionel # 33. It became my first restoration project. Here is a pic:
More of my modest "collection", some original, some restored:
Despite the fact that mine are between 89 and 110 years old, they all run great. These are the trains that I usually run around my Christmas tree.
Standard gauge track is 3 rail, just like that still used today, except that it is much wider at 2 and an 1/8 inches between the outer rails.
You can see the comparison in this photo:
Standard gauge trains remain popular, so much so that at least two major manufacturers, Lionel and MTH Trains, reproduced many beautiful examples over the last 20 years or so. Some with modern electronics and even digital sounds.
As a side note, late in what is called the "PreWar" period (prior to World War II) Lionel and American Flyer produced tin electric trains that ran on O/O27 gauge track.
The green and blue passenger sets shown below, and the freight set shown on the bottom shelf in the 2nd photo, are examples of Lionel PreWar O gauge from my collection.
On the top shelf below you can see my American Flyer PreWar O gauge:
Click here to learn about my restoration process: