I am arriving at Charles de Gaulle airport at 7:40 am on September 4, and want to get to Montparnasse to get a 9:40 am train. Is that possible?
I’d say no.
- you are arriving from a Schengen Area airport,
- you are seated in the first row of the plane,
- your plane arrives on time and gets a jet bridge (as opposed to a remote stand and having to take a bus),
- you don’t have checked luggage,
- you arrive at Terminals 2C, 2D, 2E (K gates) or 2F,
- you know the layout of the airport, of Denfert Rochereau and Gare Montparnasse,
- you are a fast walker,
- you are ready to take crowded trains (it's going to be rush hour)
- and there are no disruptions whatsoever,
then yes, it’s doable, just barely. Count 30 minutes to get from aircraft to the RER station, 10 minutes to buy a ticket, a few minutes to wait for the train, a bit under an hour for RER B to Denfert-Rochereau and metro line 6 to Montparnasse-Bienvenüe, and 10 more minutes to your train. It will be very, very tight. Do NOT take line 4.
If any of those conditions are not true (and many are not under your control), then just forget about it. So unless you have a train ticket you can change for free and just want to try to get that train and can easily switch to a later one if you don’t, I wouldn’t.
If you have checked luggage, need to go through immigration, land at a different terminal, are seated at the back of the plane, don’t even bother, book a later train.