Narrow track gauges made building railroads less expensive. In addition, narrow track gauges allowed tracks to be built more quickly and easily on narrow routes. But broad gauges also had their advantages: better running properties of the train, higher load capacities even on poor ground, and higher speeds.

The "narrowness" of a railway is defined by its "gauge," which is the distance between the inner sides of the two rails. The most common gauge in the world is the Standard Gauge of 4 feet 8.5 inches (1,435 mm). The reason for this specific measurement is largely historical and traces back to early English coal wagons and the tramways that preceded steam locomotives. Legend—and some historical evidence—suggests that this width was based on the ruts left by Roman chariots, which were built to accommodate the width of two horses' backsides. While this is often debated, it is a fact that early railway engineers like George Stephenson adopted the existing wagon widths used in English mines. In regions where "Narrow Gauge" (anything less than standard) is used, such as in mountainous areas of Switzerland or the outback of Australia, it was chosen because narrower tracks are significantly cheaper to build, require less land, and allow for much tighter curves in difficult terrain. Conversely, some countries like Russia and Spain use "Broad Gauge" to allow for larger, more stable trains, though this makes international rail connections more difficult due to the need for wheel-base adjustments at borders.