Do you want to travel in comfort within the Netherlands on the Amsterdam Centraal - Utrecht Centraal - Arnhem Centraal route? Then you will have to pay a supplement in addition to your domestic ticket, both in 1st and 2nd class.
Buy this supplement in advance on NS.nl, at the NS ticket machines or a NS service desk.
If you have a NS season ticket you can travel on ICE International within the Netherlands without having to pay the supplement.