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the métro is a very dense network, it has stops every 90 seconds and covers mostly Paris and inner suburbs. Métro lines are numbered 1 to 14. The RER has fewer stops and covers longer distances into the suburbs. The lines are lettered A to E. http://www.ratp.fr/plan-interactif/carteidf.php?lang=uk
Métro = the thin lines on the map. RER = the thicker lines on the map.
The transilien trains only leave from the major train stations and take you to distant outer suburbs although some lines have a few stops in inner suburbs too. They departt from Gare Saint-Lazare, Gare du Nord, Gare de l'Est, Gare de Lyon and Gare Montparnasse. They are represented by double lines on the map.
When you travel in Paris proper, you can take either the métro or RER, depending on which lines are more convenient for you, it's all one system so you can transfer from a métro to an RER and vice versa on the same ticket.
Edited: 9 years ago