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I have very little luggage and am looking to go by train. I'm finding the rail network highly confusing and would appreciate help. Also, what's the fare?

The Paris transport operator, RATP has a journey planner on their website. You can input your start point, end point, and start time, and it will suggest routes to you. If you don't want to worry about the bus, you can restrict it to rail (so Train, RER and Metro) only. Make sure you search for Aeroport or similar as your start point however, as France is full of places named after Charles De Gaulle. For your journey, the exact answer depends on the time of day you arrive, but taking the RER B to Châtelet-Les Halles and then the Metro line 7 towards Mairie d'Ivry looks the most straightforward. Châtelet is a big and confusing interchange however, so be ready for that.

Broadly speaking, there are several main components of the Paris transport system:

• 
  RER: this is the light rail system that links the suburbs of Paris (including Aéroport Roissy-Charles De Gaulle) and the city. It runs relatively fast, goes through very deep tunnels in the city itself, and each line will only have 3-5 stops within the city of Paris. The lines are given letters as names (the one that goes to CDG airport is B). Arriving at CDG airport, you will see signs in the airport (after bag collection) saying "Paris by Train" - these will take you to the RER stop.


• 
  Metro: this is the light rail system within the city, a conventional "subway" or "underground" type service. Services and stops are very frequent. As the stops are often close together (in many places you can see the next platform from the one you are waiting at) it often doesn't make sense to use the metro unless you are travelling quite a few stops. Most metro lines either don't leave the city proper, or only travel a very short distance outside. The metro is probably the most used service by tourists. Metro lines are identified by a number. Pierre et Marie Curie is on line 7
  


• 
  Bus: these are buses. There are huge numbers of them, and they go everywhere. Services like Roissybus run from the Airport to Opera. Buses can be intimidating to tourists, as you can't readily see on a map where they go and are subject to Parisian traffic. However, they can be both the cheapest and quickest way to get about (CDG - Gallieni was much quicker by bus than RER + Metro for example). Buses are identified by number (mostly), with numbers of up to three digits.


• 
  Tram these are mounted on rails, but mostly sharing road space with other vehicles. They largely run "around" the ring of Paris, as opposed to the RER and Metro which largely run radially from the city centre. Trams are denoted by a T and a number.

You can see Trams (dashed lines), RER (thick lines), Metro (thin lines) and mainline train services (double lines, linked) on the RATP online map (You can see CDG airport to the north-east). RATP also provide downloadable maps, and paper maps once you get to Paris.

There are a large amount of ticket options available. For the sake of your specific journey, you are probably best off buying a single ticket from the airport, to your destination, from a machine or office at the airport station. For future journeys, you may find a standard ticket, or a pass useful, but as CDG is a long way out, a pass that covers that journey to is probably bad value. You can read a full description of tickets (in French) here, including the "Billet Origine-Destination" that you probably need on the RATP website. There is an English language page, but it only describes some of the ticket types.

The quickest way seems to be :

1. From the airport, take RER B, get off at the "Châtelet-Les Halles" station


2. From Châtelet-Les Halles, take the subway 7 going to "Mairie d'Ivry" and get off at "Pierre et Marie Curie"

It should take about 1h15, price should be in the vicinity of 15€ (need confirmation).

Be aware that Châtelet-Les Halles is a pretty big station and that you might walk a bit.