Hello, I will be traveling this Friday from Rennes, France to Vienna, then to Munich and then back to Rennes.
However, I cannot book my seats and I am starting to get a little bitt worried.
What can I do in this case?

Hello, I will be traveling this Friday from Rennes, France to Vienna, then to Munich and then back to Rennes.
However, I cannot book my seats and I am starting to get a little bitt worried.
What can I do in this case?
Best answer by Angelo
You only need a reservation for the TGV Rennes to Paris and the Nightjet Paris to Vienna.
And a reservation for the ICE Stuttgart to Paris and TGV Paris to Rennes. For all other trains you can get on the train without a reservation but you can buy one the be shure to get a seat (it should be not so full due to the current situation). The ICE to Paris has only a small number of reservations for Eurail users, buy it asap.
Are you currently in France? Then go to the next train station and buy at a ticket office or automatic ticket machine of SNCF. Please buy the reservation also for your trip back to Rennes.
You can also buy it online via Eurail Reservation Service, SNCB/NMBS International (via Railplanner App [More>Seat Reservation>France] and for the Nightjet also via tickets.oebb.at (manual linked by rvdborgt)
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