I am traveling from Amsterdam to Frankfurt and I see on the app that is says reservation remommened. Do I reserve on the app or is it thru a diferente site? Do I really need to reserve? Traveling next monday morning have 1st class global pass and we are 4 people.
+15You can reserve via Railplanner 
or cheaper via bahn.com or their app DB Navigator :)
DB charges 5,60€ per Journey and person so if you go further than Frankurt you can reserve the entire way for example Munich or Vienna
Railplanner charge per train/pers :)
The reservation is optional but as amsterdam is quite popular the train might be full :/
Would you use these same sites to reserve seat from Zurich to Paris?
I'm currently in the ICE from Amsterdam to Frankfurt and I'd definitely recommend a reservation, even 1st class is almost full.
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