To reach Saint Malo from Paris by train, there are two possibilities:
i) The most comfortable solution is the TGV (Train à Grande Vitesse, ie. High Speed Train) Paris - Saint Malo. However, there are only 2 or 3 departures a day.
ii) The second choice is the TGV Paris - Rennes (departure every 30 minutes, 2 hours trip), and then, take a train from Rennes to Saint-Malo (departure every hour, 50 minutes trip). It is exactly the same travel as i), however, you will have to get off the TGV at Rennes, change platform, wait for the Rennes - Saint Malo train...
The trains Paris - Saint Malo or Paris - Rennes leave at railway station ‘Gare Montparnasse’. Be careful, there are several railway stations in Paris!
Here are the details to go from CDG airport to Gare Montparnasse:
- Once in the RER suburban railway station near the airport. Buy a ticket for Paris. This ticket allows you to go to whichever station in Paris, provided you do not go out the subway network until you reach your final destination. So, keep it in your pocket since you will need it along your journey.
- Take the RER B suburban train to go to Paris.
- Stop at station ‘Denfert-Rochereau’ (~ 50 minutes).
- Take the ‘metro’ (subway) line 6 (the light green one) towards ‘Charles de Gaulle Etoile’. Use your ticket to pass the automatic doors (You do NOT have to buy another ticket).
- Stop at station ‘Montparnasse Bienvenüe’ (5 minutes).
- Walk following the signs ‘TGV Grandes lignes’ (at the end of the platform, after some stairs, take right). After the automatic doors, walk 10 meters and get upstairs with the escalators. Go upstairs again with the escalators. Take right.
- Look at the central departures panel to learn your platform’s number. Find the platforms just behind the central departures panel. Validate your train ticket with one of the red-orange machines before taking the TGV (You may get fined if you do not, even if you have a ticket).
A map of the subway network in Paris: PDF