The RER Line B suburban train takes you from Paris-Charles de Gaulle (CDG) Airport to central Paris in as little as 35 minutes on the express trains. It is the fastest and almost-cheapest way to travel, in adequate comfort when it's not crowded, even if you have a lot of luggage.
Board any RER train on Track (Voie) 11 or 12 and it will take you to central Paris.
You'll go faster and more comfortably, with less chance of crowding, on a train that makes fewer stops. Here's how to choose one:
On monitors listing the trains, read from left to right. First comes "RER" (for Réseau Express Régional), then "B" for Line B, then the 4-letter route code identifying the train's route and stops.
The fastest trains from CDG to central Paris have route codes beginning with KA (KALE, KALI, KAPE, KARE, etc.) or JA (JAMI, JANO), or UL. Other trains make more stops. These trains stop only at Aérogare 1 (Roissypôle) and perhaps the Parc des Expositions before entering central Paris at the Gare du Nord (Paris Nord RER).
(For your return to CDG airport from central Paris, look for B3 trains with route codes beginning with E, because all trains beginning with E go to CDG. The express trains are ERIC, EROC, ERIO, ERSE, EDME OR EDDY. See below for more.)
Next on the monitor comes the departure time (in 24-hour time), the destination and perhaps some station information, and finally the track number (11 or 12).
If you're going all the way to the Antony station to take the OrlyVal shuttle train to Orly Airport, you must board an RER B4 train to Massy-Palaiseau or Saint-Rémy-lès-Chevreuse, not a B2 train going to Robinson.
Look for signs: PARIS PAR TRAIN - PARIS BY TRAIN - PARIS VIA TRENO
Insert your ticket in the turnstile, retrieve it when it pops up on top, keep it with you—you have to have it to exit the station at the end of your journey—and descend by stairs or elevator to the tracks.
Check the monitor for the track (voie) number and the 4-letter route code for your train, check the clock for the departure time, board your train and ride. (If you don't care about finding an express train, you can board any train on Tracks 11 or 12 at any time—all go to the central Paris stations.)
Called PARIS NORD on the signs in the RER station at that stop, this is a major SNCF intercity and international train station.
On the Right Bank of the River Seine in the very center of Paris near the Louvre, Hôtel de Ville and Le Marais, this station is the heart of the Paris Métro system. It's the largest Metro station in Europe—vast and confusing.
The very heart of Paris on the Left Bank near Île de la Cité, Notre-Dame, the Latin Quarter and the Sorbonne.
Near the Jardin du Luxembourg, Église de St-Sulpice and Panthéon in the Latin Quarter.
This intersection of major boulevards in the southern part of Paris is an important transport nexus, with connections to Paris-Orly Airport.
If your hotel or apartment is in the center of Paris, you'll probably want Châtelet-Les Halles, or St-Michel Notre-Dame, but from any central Paris station you can transfer to the Métro, a city bus or a taxi (minimum fare 7€) to reach your final destination.
Remember: All RER trains from CDG Airport go to central Paris, and hold on to your ticket. You'll need it to exit the train station in central Paris.
Going to the airport from central Paris, board any RER B 3 train, destination Aéroport Charles-de-Gaulle (CDG). Remember, all trains with route codes beginning with E go to CDG.
The first station stop at the airport is Aérogare 1 - Roissypôle, for hotels, Terminal 3, and the bus terminal. The second and last stop (terminus) at the airport is Aérogare 2, for Terminal 2 and the TGV train station.
Do NOT board a train with a destination of Mitry-Claye!
The RER B3 train line divides at Aulnay-sous-Bois, with one branch going to Aéroport Charles-de-Gaulle and the other to Mitry-Claye. If you board a Mitry-Claye train by mistake, get off the train at or before Aulnay-sous-Bois and wait for the next train with CDG airport as its destination.