In the 19th century, wen rail travel was beginning to become popular, railroads were not standardized. Railways decided the gauge or the distance between the two rails. This affected how wide the rolling stock (cars and engines were) Although the “standard” gauge of 4 feet and 8.5 inches is now common throughout the country, for most of the 19th century, gauges varied by railroad. For those shipping goods, their goods had to be unloaded and reloaded between railways at the change of gauge. It was an expensive and time consuming process. After the Civil War, railways throughout the South, which used a wider gauge decided, over a weekend to switch 11,000 miles of track from the Southern gauge to a gauge more in line with the rest of the country. They didn’t switch to the standard gauge but to a gauge more similar to a Pennsylvania railroad that they all connected with. For passengers and others, the switch wasn’t a big deal but it was an amazing feat of planning and engineering to switch all the rails and all the rolling stock in a matter of days.
This helpful video from The History Guy tells the whole story of exactly how they did it.