If arriving at CDG airport with family, heading for Gare du Nord, is it possible to buy RER single-trip tickets in advance to save delays and currency problems at the airport?
(Congratulations on your excellent site, by the way)
Only multi-use Paris Metro/RER passes can be purchased online ahead of time. Single use RER tickets such as for the RER B from CDG to Paris, can only be purchased at ticket windows, offices and vending machines within Paris.
(Thanks for the kind words about the site)