Why the city of Chongjin cut off from the rest of the country as noted in the book, Nothing to Envy?

According to the author, Chongjin is a highly industrial city in the north of North Korea that is cut off from the rest of the country by treacherous mountain passes. These passes lead to very infrequent railroad traffic and nearly no car traffic entering the city from surrounding areas, thus the people of Chongjin are highly isolated, self-reliant, and different from the rest of the country's population.