https://www.ns.nl/en/customer-service/lost-or-stolen/lost-and-found-items.html

The trainconductor checks for lost and found items and will report it. Follow the instructions and Contact Customer Service.

Hello Sy, welcome to the NS Community!

This link sends you to the Lost&Found page of the NS. You can notify them to be on the lookout for your item so they’ll send you a message if it’s found. Usually after 3 days. If not, it’s recommended to contact them again after 3 days. If it has been found and not collected yet after 5 days, it will be sent to the Bureau Gevonden Goed (basically National Bureau where everything gets deposited that’s Lost&Found) 

Good Luck and hope this was helpful.

Ah it seems Thom had included an English Version of the site. Might seem more appropriate.