A railway (US: railroad) is a form of transport in which vehicles, usually coupled together in groups, travel on rails along fixed routes.
The gauge of a railway is the distance between the inside edges of the top of the rails. Trains built to operate on a railway of a given gauge are unable to operate on railways of a different gauge (unless special provision is made to allow this).
The history of railways is replete with railways being built to a variety of gauges, which has led to numerous problems including limiting the viability of individual railways.
A gauge of four feet eight and a half inches is generally known as standard gauge, and is widely used around the world. Gauges smaller than this are referred to as narrow gauge and larger gauges are referred to as broad gauge.
Narrow-gauge lines are usually cheaper to construct and have often been used for short industrial lines, connecting branch lines, and places where it would not be economic to construct a more substantial line. However, some narrow gauge lines, such as those of Queensland and South Africa, are built to the same standards as standard gauge lines and perform just like standard-gauge lines.