Narrower gauge railways usually cost less to build because they are usually lighter in construction, using smaller cars and locomotives (smaller loading gauge), as well as smaller bridges, smaller tunnels (smaller structure gauge).

Most of the world's railways use "Standard Gauge," which is exactly 4 feet, 8.5 inches (1,435 mm) wide. The reason they aren't wider is largely due to historical momentum and the "Path Dependency" of engineering. This specific width was popularized by George Stephenson for early British coal railways, which were themselves based on the width of pre-existing horse-drawn wagons and the tramways used in mines. There is a popular (though slightly exaggerated) legend that this width traces back to Roman chariot ruts. While a "Broad Gauge" (wider rails) would actually allow for much faster, more stable, and higher-capacity trains, the cost of converting thousands of miles of existing track and rolling stock is prohibitive. Some countries, like Russia and Spain, do use wider gauges, which requires "gauge-changing" trains or passenger transfers at borders. For most of the world, we are "stuck" with Standard Gauge because it makes international trade and equipment manufacturing far more practical and cost-effective than building wider, incompatible lines.