According to Wikipedia:
"In England some early lines in colliery (coal mining) areas in the northeast of the country were built to a gauge of 4 ft 8 in (1,422 mm)...
George Stephenson used the 4 ft 8 1⁄2 in (1,435 mm) gauge (with an extra half-inch of free movement to reduce binding on curves) for the Liverpool and Manchester Railway, authorized in 1826 and opened 30 September 1830. The success of this project led to George Stephenson and his son Robert being employed to engineer several other larger railway projects...
The influence of the Stephensons appears to be the main reason that the 4 ft 8 1⁄2 in (1,435 mm) gauge became the standard, and its usage became more widespread than any other gauge."
Thank you, Wikipedia, and Norviews for the context
