Rail Gauges

Rail gauges present another set of differing national standards that are really little more than historical accidents. Also, this is relevant to the difficulties faced by Ukraine in trying to export farm goods by rail.

World Rail Gauges (from here).

Prior to 1886, the South and the North in the U.S. had different rail gauges, but there is now a standard 1435 mm standard, which is the same as the "standard gauge" in Europe.

The vast majority of North American railroads are standard gauge (4 ft 8+1⁄2 in / 1,435 mm). Exceptions include some streetcar, subway and rapid transit systems, mining and tunneling operations, and some narrow-gauge lines particularly in the west, e.g. the isolated White Pass and Yukon Route system, and the former Newfoundland Railway. . . . 

In 1886, the southern railroads agreed to coordinate changing gauge on all their tracks. After considerable debate and planning, most of the southern rail network was converted from 5 ft (1,524 mm) gauge to 4 ft 9 in (1,448 mm) gauge, then the standard of the Pennsylvania Railroad, over two days beginning on May 31, 1886. Over a period of 36 hours, tens of thousands of workers pulled the spikes from the west rail of all the broad-gauge lines in the South, moved them 3 in (76 mm) east and spiked them back in place. The new gauge was close enough that standard gauge equipment could run on it without problem. By June 1886, all major railroads in North America were using approximately the same gauge. The final conversion to true standard gauge took place gradually as track was maintained. Now, the only broad-gauge rail systems in the United States are isolated rapid transit, light rail, and streetcar systems as well as some heritage railways and hill railways.

Quoted material from Wikipedia.