If you have some time to spare-and save the money-it has been told here so many times-hop over border by local train (Offenburg-Strasbourg). DB Resiecenter will open (guess) by 7.00-but chance they can still get that place seems minimal to me-as there are limited Quota for passes. You will even have much more with that to do in case you have not yet made any for onward from Paris-which is a detour anyway for this trip (use direct TGV Strasbourg to south of FRance, like Avignon or Perpignan-then local train onward. Forget any idea about the direct TGV P->Bcn. Plus that it cost an arm+leg extra.
your overnight train will be late anyway on arrival in KA-stay on it till that Offenburg. The local bakery has good offer+cf from 4.30 in the morning in the small shop beside exit
You can use local trains Karlsruhe-Strasbourg wich are without reservations.
and then the first TGV train to Paris from Strasbourg.
You can book online via interrail.eu or B-Europe https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish you need a paper pass cover number (if you have a mobile pass request it here:
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Within France you can get a reservation for an TGV until the train is full booked. You can also buy it in Strasbourg ticket office or ticket machine.
If you really want direct TGV DE-FR, there’s a once a day connection Freiburg/Offenburg to Paris. this can typically be reserved late notice (still available for today) and at lower cost (10 euro)
