At Porte d'Orléans, you'll be able to take the bus, the subway and the tramway. With a lot of options, you'll be able to reach quickly and easily all the touristic and commercial spots of Paris.
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At SUBWAY's station Porte d'Orléans, you'll take the fourth line which will directly bring you to Montparnasse (where there's the train station as well), but also to Saint-Michel, Saint Germain des prés, the Cathédrale de Notre-Dame de Paris, to the Centre Pompidou, etc.
At Porte d'Orléans, you'll have access to THREE DIFFERENT BUS LINES, which will directly bring you to the most touristic spots of Paris :
* line n°38 : it will mostly lead you (map's A zone) to the Luxembourg, the prestigious district of Saint Michel - Saint Germain, the Town Hall, the Centre Pompidou, etc...
* Line n°68 : it will mostly lead you (map's B zone) to Rue du Bac, Musée d'Orsay, Musée du Louvre, Palais Royal, and then (C zone) to the Opera de Paris.
* Line n°28 : it will mostly lead you to Montparnasse (the district and the train station), the military school and the Eiffel Tower, (E zone) and (D zone), the Invalides, the Palais de la Découverte, the Champs-Elysées, Haussmann's area, etc.
By taking the bus, it will usually take you 20 to 30 minutes to get to these different important spots of Paris. And it's a very nice way to discover town.
Finally, still at Porte d'Orléans, you'll also have access to the TRAMWAY. It goes through the entire south of Paris, from the Pont du Garigliano (west) to Porte d'Ivry (east).
RATP's website (the company in charge of the subway and the bus) will give you all the informations you might need.
You'll find more information on public transports and the different kinds of pass (transportation, visits) at your disposal on the Tourism's section.