RER lines in Paris

Yes, as long as you are inside the inner two zones of the Paris Metro system. You do need your t+ ticket or pass to exit the RER system. This is to make sure you did not enter the system outside the central zones, in which case you would owe more money.

If say you bought a ticket at CDG and wanted to go to Gare de Lyon. You paid 9.75 euro for the ticket. You can then change to the A line at Chatelet, then exit at Gare de Lyon after scanning your ticket to open the turnstile. The system computer immediately knows you paid the right fare. If on the other hand, you staid on to Disneyland, the turnstile would not open until you bought the proper fare adder.

often RER changes are literally just a matter of walking across the platform. Once you are in the RER station at a metro/RER station you can transfer to any other line. RER is tricky in Paris since you can only use it in zone 1 for a t+ (ordinary metro) ticket. e.g. if you are taking the metro line 1 to la Defense which is outside Paris you can do this with an ordinary ticket but if you go to the same station on the RER it costs more and you risk being fined if you arrive on the ordinary ticket. An ordinary t+ ticket is good regardless of zone everywhere the metro goes.

Coming into Paris on the RER (which costs 10 Euro at least now) if you need to transfer to the metro, you would probably exit the RER and then re-insert the ticket into a metro stile. The system knows you are in transit. RER trips are metered so you usually have to use the ticket to enter and exit (they are metered because different trips require different tickets); the metro is one size fits all so you only need the ticket to enter.

Be sure you hang onto the validated ticket until you completely clear the system. If you are asked by an inspector to show your ticket and you don't have a valid ticket (they have a device that reads it) then there is a large on the spot fine. This doesn't happen often, but it happens. Last time we were in Paris for a month, we were checked 4 times, usually entering a platform or exiting the system.

Yes, hang onto the ticket! Our first two trips no problem, last trip we were checked twice....both at Gare du Nord.

To exit the RER you must feed your ticket into the machine to get through the turnstile. That's why you have to keep the ticket. In the metro hold on to the ticket because inspectors might ask to see it. You don't need it to exit like you do on the RER.