No reservation is required for the ICE, no supplement either if you're using a pass. There are no EC trains in Belgium and the Netherlands and domestic IC trains don't have reservations.
For international journeys, you can book seat reservations for ICE and the IC Berlin-Amsterdam. Should be free of charge in 1st class via tickets.oebb.at (use Interrail as a discount NOT reservation only), otherwise €3 per train. Alternatively, you can book seat reservations via bahn.com (Seat only) for €4.50 per journey, so including any bookable connecting trains.
@rvdborgt I tried to book a free reservation via ÖBB for 1st class but via APP I can't do it. I think it is only if you buy something and you get the reservation on top for free.
@rvdborgt I tried to book a free reservation via ÖBB for 1st class but via APP I can't do it. I think it is only if you buy something and you get the reservation on top for free.
For which train? (route, date, departure time)
I did a few test bookings a few days ago for trains in Germany and it worked, using the discount method. For some trains it didn't and it just gave "No ticket available”. That was using the website.