Originally Posted by
pepe C
I am not 100% sure, but I think it is required that you get a stamp on your ticket in the train.
On the way TO Amsterdam, yes; you can only check in for your onward flight at the KLM desk for these train tickets - online/mobile/app/kiosk checkin is disabled - and, anxious to protect the integrity of these train tickets, they ask to see your stamped train ticket before unblocking the flight.
On the way FROM Amsterdam, you should exchange the flight coupon for a train ticket at the NS desk, but there is no further requirement to actually travel.