To commute between Orly airport and CDG airport in paris you have 2 main options ;
Trains ( RER line B ) , Taxis .
RER line B (suburban Train):
There is « RER B » that goes from the center of Paris to « Antony » train Station, there you have to take another special automated train named « Orlyval » to airport. The ticket cost 12-13 €. travel time is at least 35min. Train intervals are 3-4 per hour.
Since RER line B almost connect two airports ;
So you can come the same way back to Paris and continue To the CDG airport, once you arrive in either « Chatelet » or « Gare de Nord » train station, make sure you be in right train for CDG! Not all RER B trains go to CDG. Travel time is about 75 min.
If you have a family or many luggages with you trains and buses for Orly are not a good choice for you, it will be difficult to carry all that through stairs and crowded trains, buses and changes etc..
Is the easiest and surest way to commute between Orly and CDG,travel time is about 50 min (in normal traffic) and cost about 60€.
During the night or early morning taking a Taxi is recommended, rather than trains !
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