One reason is that China is geographically closer to North Korea than the United States is. Also, both China and North Korea are communist countries, and China fought with North Korea during the Korean War, while the United States fought against North Korea.
Unlike China, the United States has threatened North Korea multiple times. From promising to "nuke" them, to spreading things about the country that may or may not be true, the United States has not been friendly to North Korea. China and North Korea, however, trade often. Therefore, North Korea would be closer to China than the United States. I hope this helps!
The development of railroads was one of the most important phenomena of the Industrial Revolution. Trains provided supplies and equipment and a lifeline of the army. Trains made a big impact on the citizens. Railroads were visible symbols of industry and modernity during the Civil War.
