Proposed in 1934 by geographer Hu Huanyong, the Heihe-Tengchong line is not China’s newest highspeed rail line, but an interesting artifact of human geography. This line divides China roughly into two halves  in terms of geographic area (57% to the West, 43% to the East), but nearly all the population - a whopping 94% - resides to the East of the line. 
Impressive though these numbers may be, what’s perhaps more impressive is that despite having a population density lower than all but 27 countries, the western half of China would still be the 16th largest country in the world by population, just below Germany. 

As you can see from the map, China’s population density is highly concentrated between the Yellow River (黄河) to the north and the Yang-tze River (长江)to the Souh, as well as along the coast. The large red spot just east of the line represents a very fertile agricultural area sometimes referred to as “China’s breadbasket” and includes the megacities Chengdu and Chongqing.  Most of China’s West is arid, high up in the mountains, or both, making it difficult to sustain dense populations.