There is no supplement required for this, it was an additional charge only in July and August this year.

Add to planner and use any unreserved seat or if you wish reserve seats at bahn.com or oebb.at

Many thanks for your help! I hope you are right, but the app yelling at you doesn't inspire confidence in not getting kicked off the train and fined!

I'm also having a look at some trains between Amsterdam and other cities in the Netherlands and Brussels on IC trains, and the app is saying supplement needed again. On the NS website it says that the 2.90 Euro supplement is only for airport travel? Is this another case of the app not showing the correct info?

Apologies for the paranoid questions, first time, hoping the trip goes smoothly!

There is no supplement for passholders, the app is again wrong. It had been corrected but the mistake is back…

Seat reservations are not possible either in Belgium and the Netherlands. You hop on and sit wherever you want. As a tip : train ends are generally less busy.

More info about seat reservations in general (though it is wrong too on the NS supplement) : https://www.seat61.com/interrail-and-eurail-reservations.htm

I've taken that route twice in August and they never asked about some sort of supplement.

Thanks for your help. Fingers crossed I won't get checked either.