The Days They Changed the Gauge

Tracks in the South had been built with a gauge (or track width) of 5 feet but the majority of tracks in the North had a 4-foot 9-inch gauge (more or less). So after much planning, over a concentrated two-day period in the summer of 1886, the width of thousands of miles of railroad track (and the wheels on thousands of rail cars) in the South was reduced by three inches.
— Read on kottke.org/14/09/the-days-they-changed-the-gauge