Four Feet, Eight and a Half Inches

The rails under almost every train on earth sit four feet eight and a half inches apart. Nobody designed that number. George Stephenson inherited four feet eight inches from the Killingworth colliery wagonway and added the half inch around 1829, most likely for flange clearance on curves. The reason was never properly written down.

Isambard Kingdom Brunel refused the inheritance. The Great Western Railway ran on seven feet, a gauge chosen for stability and speed rather than for what colliery carts had happened to measure. Two systems grew across Britain until they met, and where they met, every passenger and every ton of freight had to change trains.

The Royal Commission on Railway Gauges was appointed to settle it. Its report recommended four feet eight and a half inches to end the breaks of gauge, while conceding advantages to the broad gauge in fast passenger work. In the Lords on 24 February 1846, Lord Kinnaird said it "appeared to him to be in favour of the broad gauge, except for the conveyance of merchandize".

The Railway Regulation Gauge Act received royal assent on 18 August 1846. Future passenger lines had to use the narrow gauge, with specified exceptions protecting Great Western territory. The deciding argument was never that the winner ran better. It was that the winner already covered more ground, and mixed systems punish everyone at the junction.

The erasure took decades and then one weekend. From 1866 the Great Western converted its network piece by piece, and on 21 and 22 May 1892 roughly 4,200 platelayers lifted about 171 route miles of the last broad gauge. Brunel's seven feet is gone. Stephenson's undocumented half inch is under every train you have ever boarded.