Train travel in Europe always makes me a little nervous. I don't think we will purchase our tickets prior to our trip, but am wondering about train travel from CDG to the Monparnasse station which is near our hotel. Since we will have just arrived and won't have our barrings yet, I want to make sure that we get the right train and know what stop to look for. Does anyone know how long the train ride is and what the approx. cost would be? Thanks.
Take the RER "B" express (not the local) from CDG to the Denfert-Rochereau station in Paris, then transfer to the Metro "4" line (going toward Portt de Clignancourt) to the third stop, Montparnasse-Bienvenue. That Metro stop is north of the railroad station, there's a long ride on a moving walkway to the station. The trip should take about an hour at midday, most of which will be on the RER train. www.parisbytrain.com is a useful website for all things rail, including finding where the trains are, when they go, what they cost etc. It has a route finder for specific itineraries, and links to the different rail systems (RER, Metro, SNCF, etc.) Also maps. No need to get tickets before the trip, I'm not even sure you can. If you have luggage, kids, or other reasons to prefer an easier ride, you could try the Air France shuttle bus between CDG and Gare Montparnasse, making only one stop (Gare de Lyon). See www.lescarsairfrance.com, it's also described on the parisbytrain.com site. This bus leaves CDG every half hour, takes a little longer than the RER/metro combination, and costs more (but way less than a taxi). But you get a comfortable seat, your baggage is secure (under the bus), and you see more of Paris on the ride.
Just adding that the Air France bus makes only one stop (Gare de Lyon) before Gare Montparnasse after leaving the airport.
A ticket from CDG to anywhere in central Paris on the RER and Metro costs €9.50. It provides unlimited transfers until you exit the system. All RER B line trains use the same tracks. Unless the next train happens to be an express, taking the first available train will get you to Paris faster than passing by regular trains in favor of waiting for an express. For a zoomable map of the Metro and RER system, go here: http://tinyurl.com/6xmvms4.
If you need to get to the Gare Montparnasse area and are nervous about trains, I highly support the idea of taking the AirFrance bus directly to Gare de Montparnasse (with the one intermediate stop at Gare de Lyon). So much more comfortable, no schlepping luggage around train stations, worrying about getting pick pocketed on the RER, etc, and direct to your destination. One hint: for some reason buses are called "cars" in France, so upon arrival at CDG, look for the transport signs directing you to Cars AirFrance. If you google them, you'll find all the information, in English.