As railways developed and expanded one of the key issues to be decided was that of the rail gauge (the distance between the two rails of the track) that should be used. The eventual result was the adoption throughout a large part of the world of a standard gauge allowing inter-connectivity and the inter-operability of trains. The distance between the inner sides of the rails in this gauge is 1435 mm (4 ft 8½ in). Currently 60% of the world's railway lines are built to this gauge. It is also named Stephenson gauge after George Stephenson.
In Great Britain the standard gauge was at first 4 ft 8 in (1422 mm) but it was soon widened slightly. Parts of the United States rail system, mainly in the northeast, adopted the same gauge because some early trains were purchased from Britain. However, until well into the second half of the 19th century Britain and the USA had several different track gauges. The American gauges slowly converged as the advantages of equipment interchange became more and more apparent; the destruction of much of the South's broad gauge system in the American Civil War hastened this trend.

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