It achieved royal assent on 18 August 1846, during the reign of Queen Victoria of the United Kingdom of Great Britain and Ireland.

It mandated that the track gauge – which was the distance between the two running rails' inner faces – of 4 feet 81⁄2 inches to be the standard for Great Britain and 5 feet 3 inches to be the standard for Ireland.

[2] The act continued legislative approval of the broad-gauge railways constructed by the Great Western Railway engineer Isambard Kingdom Brunel and endorsed the construction of several new broad-gauge lines, but restricted them to the south-west of England and to Wales.

The act stated that these railways "shall be constructed on the Gauge of Seven Feet".

[2][note 2] The resulting isolation of these lines ultimately contributed to the demise of the Great Western Railway broad-gauge system.