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!
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!