Introduction to unified rails: During the Warring States Period, roads in various countries were different in width and width, there were many types of vehicles, and rail sizes were chaotic. After the unification of the Qin Dynasty, not only did roads of unified specifications be built, but the uniform width of the track was also stipulated to be 6 feet. In the Qin Dynasty, a foot was equivalent to 23.1 centimeters today, and 6 feet was equivalent to 138.6 centimeters today. After the train tracks are unified, no matter what type of vehicle they are, they can travel unimpeded on traffic avenues across the country.