Most cities have train tracks which cut through them; it's nothing too special. But there aren't many cities in which the tracks are basically just another street, with houses and businesses lined up right along the side. Hanoi doesn't have time for your conventions or common sense! If there's a place in the old town where someone can live and work, someone will be living and working there.
Of the entire 1600-kilometer train journey which unites North and South Vietnam, the most spectacular is the short segment connecting the cities of Danang and Hue, where the tracks climb over the Hải Vân Pass.