The Railway Regulation (Gauge) Act 1846^[1] (9 & 10 Vict. c. 57) or the Regulating the Gauge of Railways Act 1846 or the Gauge of Railways Act 1846 is an act of the Parliament of the United Kingdom, that was designed to standardise railway tracks. 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][note 1]}

The act stipulated that:

...after the passing of this Act it shall not be lawful (except as herein-after excepted) to construct any Railway for the Conveyance of Passengers on any Gauge other than Four Foot Eight Inches and Half an Inch in Great Britain, and Five Feet Three Inches in Ireland...

Furthermore, it also provided that, following the act's passing, it would be illegal to alter a railway gauge that was in use for the conveyance, i.e. transport, of passengers.^[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.^{[citation needed]}