Introduction: Before Qin Shihuang unified the Central Plains, there was no unified system in all countries, and the size of carriages varied from place to place, so the lanes were wide and narrow. With the reunification of the country, it is inconvenient for vehicles to take different lanes. Since then, it has been stipulated that the distance between the two wheels on the vehicle should be changed to six feet, so that the distance between the wheels will be the same. This makes it convenient for vehicles to travel all over the country. This is called "the same rail car".
Cars on the same track: the rut width is the same all over the country. Unified writing and unified estrus. Metaphor is national unity. In ancient times, they were all dirt roads. After repeated rolling, two hard lanes as wide as wheels were formed. When the carriage is transported for a long distance, the wheels are always kept in the hard lane, just like modern vehicles walking on asphalt roads, which can significantly reduce animal power consumption and axle wear. The Qin dynasty enacted the law that cars share the same track, which can make the roads in the whole country be pressed into hard lanes with the same width within a few years.