A standard-gauge railway is a railway with a track gauge of . The standard gauge is also called Stephenson gauge (after George Stephenson), international gauge, UIC gauge, uniform gauge, normal gauge and European gauge in Europe, and SGR in East Africa. It is the most widely used track gauge around the world, with about 55% of the lines in the world using it. All high-speed rail lines use standard gauge except those in Russia, Finland, and Uzbekistan. The distance between the inside edges of the rails is defined to be 1,435 mm except in the United States and on some heritage British lines, where it is defined in U.S. customary/Imperial units as exactly "four feet eight and one half inches", which is equivalent to 1,435.1mm. History As railways developed and expanded, one of the key issues was the track gauge (the distance, or width, between the inner sides of the rails) to be used. Different railways used different gauges, and where rails of different gauge met – a "gauge break" – loads had to be unloaded from one set of rail cars and reloaded onto another, a time-consuming and expensive process. The result was the adoption throughout a large part of the world of a "standard gauge" of , allowing interconnectivity and interoperability. Origins A popular legend that has been around since at least 1937 traces the origin of the gauge even further back than the coalfields of northern England, pointing to the evidence of rutted roads marked by chariot wheels dating from the Roman Empire. Snopes categorised this legend as "false", but commented, "it is perhaps more fairly labelled as 'True, but for trivial and unremarkable reasons. The historical tendency to place the wheels of horse-drawn vehicles around apart probably derives from the width needed to fit a carthorse in between the shafts. Research, however, has been undertaken to support the hypothesis that "the origin of the standard gauge of the railway might result from an interval of wheel ruts of prehistoric ancient carriages". In addition, while road-travelling vehicles are typically measured from the outermost portions of the wheel rims (and some evidence shows that the first railways were measured in this way, as well), it became apparent that for vehicles travelling on rails, having main wheel flanges that fit inside the rails is better, thus the minimum distance between the wheels (and, by extension, the inside faces of