The old paper tickets are obsolete on Dutch trains. Now you need a smart card, called the OV-chipkaart. Similar to NYC’s Metro card or the London Oyster and Travel cards, these can be bought as either single use or multi-use, reloadable cards. Next time I get to Amsterdam, I’m gonna check this out. I bought some paper tickets at Tourist info across from Centraal station last year, so there may be a ticket machine there for us wandering foreign types.
Another wrinkle on this change – you may need a card to even get onto the platform. This is already being rolled out near Utrecht… forewarned and all that…
Note – I found out about this 2 years ago, so there may be even more changes that have been implemented by now, especially with the refugee flood. Just click on the chipkaart link above for the latest news!