What the Heck is On30?
The Thunder Mesa Mining Co. is modeled in what is commonly known as "On30" - pronounced "OH-EN-THIRTY." This esoteric code is merely model railroader shorthand for the scale and gauge of what is being modeled. Scale and gauge can be confusing to the uninitiated and the terms are sometimes, regrettably, used interchangeably. Simply stated: "Scale" refers only to the relative size of a model in relation to its real world counterpart, whereas "Gauge" is a railway specific term that refers to the distance between the rails. In the case of On30, the "O" stands for O scale, which is 1:48, or 1/4" to the foot. A six foot tall person in O scale would be exactly 1 1/2" tall in our full sized world. The "n" in On30 stands for "narrow gauge." The "30" tells you that the gauge is thirty inches. To further confuse matters, 30" gauge in O scale is roughly the same as standard gauge in HO scale (1:87). In fact, On30 began as an effort to depict O scale narrow gauge by using HO scale track and mechanisms as a relatively inexpensive starting point.
Narrow gauge railroads, usually 3' between the rails, were common for mining and logging operations in the late 19th and early 20th centuries. They were cheaper and faster to build than the larger "standard gauge" railroads (4' 8.5" between the rails) that most Americans are familiar with today. Some railroads where built to even narrower gauges, such as the classic 2' roads in Maine, and there were 20" and 18" gauge railways too, usually used only in mines, industrial facilities, or plantations.
True 30" gauge railroads were actually quite rare in the United States so On30 modelers often fudge a bit and use the scale/gauge to depict 3' Colorado style narrow gauge. Then again, some go the other way and use it to model the Maine 2-footers. Since such minute differences in gauge are only important to serious builders of miniature railways, and since the primary purpose of the TMMC is entertainment, the gauge in Thunder Mesa is simply "narrow," leaving others to quibble about what amounts to 1/8" in one direction or the other.