Indeed. It's an absolute pain in real life trying to go south to Gare Montparnasse when you've just arrived at Gare du Nord - your options are: to walk, which takes forever; to take a bus, which is still rather slow and awkward with luggage; or to take the Metro, which is only a bit faster and even more awkward with luggage - there aren't any lifts at the Montparnasse Metro station!