Standard gauge
As railways developed and expanded one of the key issues to be decided was that of the gauge (the distance between the two rails of the track) which 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 1435mm (4 feet 8.5 inches). Currently 60% of the world's railway lines are built to this gauge.
In the United Kingdom the standard gauge was at first 4 feet 8 inches but it was soon widened slightly. In the United States, because some early trains were purchased from the UK, parts of the rail system, mainly in the north-east, adopted the same gauge. However, until well into the second half of the 19th century the UK and the USA had several different gauges of track. 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.
In the UK, a Royal Commission in 1845 reported in favour of the 4ft 8.5in gauge on the grounds that its network was eight times larger than the rival 7ft gauge adopted principally by the Great Western Railway. The subsequent Gauge Act of 1846 ruled that new railways should be built at 4ft 8.5in, but nevertheless allowed the broad gauge companies to continue expanding their networks.
A popular urban legend traces it even further to rutted roads dating back to the Roman Empire.
See also: Broad gauge, Narrow gaugeOrigin
There is no good reason for this particular gauge to have become the standard, other than perhaps it was more widespread than any other. In fact, many engineers have considered it less than ideal. A smaller gauge offers cheaper construction but at the cost of restricted speeds owing to reduced stability. Broader gauges are more stable at speed and allow larger, wider, heavier loads.