Traveling from CDG to Chartres by train.

You can't go directly from CDG to Chartres. You will have to go to Gare Montparnasse in Paris and then take the trip to Chartres on the local TER train.

The best way is by taxi. But you could take the RERB and then the metro 4 or 6 to Montparnasse. To me I am usually jetlagged and just want to sit and not think, so I genereally go for a cab. It's an easy process and there's usually one train an hour to Chartres.

At Montparnasse you'll want to follow the signs for the Grandes Lignes trains and generally the ones for Chartres (who may say Nogent-le-Rotrou OR Le Mans on the train) are either on the right or left of the grand hall. Trains are not reserved by seat, but there's free seating. I suggest sitting on the right side because as you're leaving there are some great views of the Eiffel Tower from afar.

I take the RER B from the CDG airport and get off at Denfort Rochereau. Transfer to #6 and go three stops to Gare Montparnasse. That is the train station to take you to Chartres.

Once at Gare Montparnasse, you will have plenty of ticket machines to buy the tickets to Chartres, and the platform will be announced and shown on the huge boards approx 20 minutes before train departure.

You mention that you think a taxi is the best. How difficult is it to get a taxi to go all the way to Chartres from CDG and what is the cost and time involved.? Would that be safe for a woman traveling alone? I would be meeting up with my RSE tour in Chartres. I'm concerned about maneuvering the trains stations and transfers by myself. Thanks for the help.

https://parisbytrain.com/gare-montparnasse-photo-tour/

Here is a photo-guided tour of the Montparnasse station which might help you feel a little more comfortable once you are there. Paris By Train is a long-standing blog with plenty of useful information.

the new sncfconnect site or app should now tell you how to get from CDG to chartres, including the public transit required.

You can buy tickets for the long distance train from Gare Montparnasse to Chartres, but you might still have to download another app to buy the public transit tickets for the RER B/metro. (This functionality is coming later, not sure when).

A taxi from CDG to Chartres should cost 200 euros or so (Uber is 170 as I type this). It feels like a lot of money for a single traveller, but perhaps you might find it worth it in the broader context of a European vacation (which does not come cheap). I am not aware of any safety issues related to taking cabs in France as a single woman.