Track gauge

The rail gauge is the distance between two rails on a rail bed of a railroad. In North America and most of Europe, the gauge has been standarized at 1435mm (4'8½"). This is known as "standard gauge." For the U.S. this number was necessary because a number of early lines were built to fit standard-gauge locomotives manufactured by English railroad pioneer George Stephenson.

For political reasons, some countries chose a broader gauge, and thus Russia and former Soviet Union countries have a wider gauge of 1524mm, as does Spain and Portugal. (Some special passenger trains have special wheel sets and can actually run through). With the advent of the European Community, Spain has embarked upon a partial regauging program.

In some cases a much narrower gauge was chosen. While this generally can't handle as much traffic, cost of construction is somewhat less expensive, and this is particularly true in mountainous regions.

A dual gauge track has three or four rails positioned such that trains of two gauges can use it; it is applied in part of the railroads of Switzerland, Australia, Brazil, North Korea, Tunisia and Vietnam.

There is a story that rail gauge was derived from the rutways created by war chariots used by Imperial Rome, which everyone else had to follow to preserve their their wagon wheels, and because Julius Caesar set this width under Roman law so that vehicles could traverse Roman villages and towns without getting caught in stone ruts of differing widths. However, an equal gauge is probably coincidence. Excavations at the buried cities of Pompeii and Herculaneum revealed ruts averaged 4'9" (1447.8 mm) center to center, with a gauge of 4'6" (1371.6 mm). The designers of both chariots and trams and trains were dealing with a similar issue, namely hauling wheeled vehicles behind draft animals.