Railway gauge refers to the distance between the inner sides of the two parallel rails that make up a railway track. This measurement is crucial because it affects the stability and speed of trains. Different countries and regions may use various gauges, which can impact the interoperability of train systems. The most common gauge worldwide is the standard gauge, measuring 1,435 millimeters (4 feet 8.5 inches). Other gauges include narrow gauge and broad gauge, which are used for specific purposes, such as in mountainous areas or for high-speed trains. The choice of gauge can influence construction costs and operational efficiency.