As the railway developed and expanded one of the key issues was the gauge (rail separation) of track to be used. This eventually resulted in the adoption of a standard gauge to allow inter-connectivity and inter-operability of the trains. It means that the distance between the inner sides of the rails is 1.435 metres (4 feet 8.5 inches). Currently 60% of the railway tracks in the world are this gauge.
In the United Kingdom the gauge was at first 4 feet 8 inches but it was soon widened slightly to the standard gauge. In the United States, because many early trains were purchased from the UK, part of the rail system adopted the same gauge. However, until well into the second half of the 19th century both the UK and the USA had several different gauges of track.
A popular urban legend traces it even further to rutted roads dating back to the Roman Empire.
See also: Broad gauge, Narrow gauge
wikipedia.org dumped 2003-03-17 with terodump