Hi, yes the Gare du Nord (ie North Train Station) mainly has trains going to the north. For the south, you'd have to go to the Gare de Lyon, Lyon being a city in southern France :)

In Moscow there's the Kazansky, Yaroslavsky and Leningradsky stations. They're next to each other but serve lines going to different cities.

From Leningradsky station you can go to St. Petersburg (formerly Leningrad); unsurprisingly, that train arrives at Moskovsky station.