|Paris RER Train Ticket Problems
The Paris Métro and RER suburban train systems are linked at some stations. Often this means you can ride the Métro, exit that system, then buy a pass or ticket from a fare booth or machine and enter the RER system.
However, at some smaller stations the systems may be linked so that you walk directly from the Métro to the RER, with no fare booth, fare machine, or ground-level exit available. You may be trapped!
If you have an RER ticket in hand, or are using a Navigo transit pass valid for both the Métro and RER systems, and for all the zones to your destination, no problem! You can easily pass the barrier between the Métro and RER systems and you're on your way.
But if you have no RER ticket, you will have to return to the Métro system and ride Métro trains until you find an RER fare booth!
By the way, the barriers that separate the Métro and RER systems are usually just that: impassable barriers designed to keep fare-jumpers from passing from one system to the other without a valid titre de transport (pass or ticket). They are not just turnstiles which you can sneak over or under. You cannot get over, around or through them, even in an emergency, and at some of the barriers there is no agent nearby to help you out of your predicament, which may be urgent because you have a plane to catch!
Buy Passes/Tickets in Advance!
Thus I strongly suggest that if you plan to ride the RER B train from central Paris to Charles-de-Gaulle Airport, you buy your RER pass or ticket the day before you travel. That way, if you happen to encounter one of these direct station links with no fare booth or machine, you'll have what you need to ener the RER system and you won't delay your trip to the airport by having to return to the Métro and ride a train until you can buy a pass or ticket.
Keep Your RER Pass/Ticket!
Also remember that you will need your RER pass or ticket to exit the RER system at your destination, including at CDG Airport. If you have no pass or ticket, you will not be able to exit—there are barriers between the RER system and the airport. There may not be an RER agent to help you, and the barriers will prevent you from entering the airport.
A Bad Experience
But even that does not guarantee a problem-free ride. On June 30, 2014, we bought our RER tickets to Aéroport Paris-Charles de Gaulle in advance, went to board the train at the Châtelet station, and the ticket was non-valable (not valid). No reason given. We were able to board anyway, but when we got to the airport, it was again non-valable to exit the station, so we couldn't get into the airport!
Luckily, we were able to summon an agent.
We made it to our flight, but it was frustrating not to have the system work as it should.