hello Ben! i would like to ask about the RER-metro using time with one single RERticket. i check the route from CDG to Kleber rue with itinerary, and do not understand if i can go with one 10,3euro ticket or do i have to buy another? (the link gives now 3 possibilities, two for 10,3eur, one for 12,50euro, i just would like to understand the reason, cos i do not understand the difference) https://www.ratp.fr/en/itineraires/A%C3%A9roport%20Charles%20de%20Gaulle%201_%2093290%20Tremblay-en-France%2612%20Rue%20Kl%C3%A9ber%2093100%20Montreuil .
Please help with it. thanks a lot!! Imola
You can use a single ticket for either itinerary, but the RER + RER itinerary will be more expensive (I'll explain why below).
The difference between the two itineraries is the usage of Paris RER + Metro (cheaper, 10,30€) or Paris RER only (12,50€).
The reason why finishing this trip using Paris Metro is cheaper is because Paris Metro is all one zone, regardless of whether a station is outside zone 1 or not. Paris Metro line 9 is one such example. It's terminus station, Mairie de Montreuil, is in zone 2, but you don't pay any extra for this using Paris Metro.
RER trains are not like this and you pay depending on zone / station location.
The RER A + RER B itinerary ends at Vincennes RER station, in Paris fare zone 2 (https://parisbytrain.com/paris-transportation-zone-map/). With RER trains, you pay "station to station". Since this itinerary goes from Zone 5 (CDG) to Zone 1 (Chatelet Les Halles), to Zone 2 (Vincennes), the extra cost is paying for the last part of Zone 1 to Zone 2 (Chatelet to Vincennes). A ticket seller at CDG can create this ticket for you and it will be a single ticket good for the entire trip.
There is much less walking at the end for the RER + Metro itinerary, so I think that's probably the better itinerary (4 min. walk for RER + Metro vs. 20 min. walk for RER + RER).