Hi there,
On the NS website - https://www.ns.nl/en/customer-service/forgotten/check-in-or-out-forgotten.html it states that:
---------------------------------------
Forgotten to check in
Many of our stations have gates that only open after you check in with your OV-chipkaart or debit card. If you're travelling from a station with poles, we understand that you might forget to check in every now and again. If you forget, try to check in at the same station anyway or find the conductor in the train to avoid a fine. After all, you always need a valid ticket when travelling on public transport.
----------------------------------------
I made a trip where the conductor kindly held the train for us but we had to run to the front of the train where she was at the only open door. Inside the train, I immediately informed her that I had forgotten/missed the check-in (post station, no gates). I was told not a problem and she checked me in on the mobile wand.
I received an email the next day informing me that I had not checked in and an EUR 50 fine had been applied.
This has since been refunded by great customer service, however they said that as soon as the conductor checks you in, it is an instant correction penalty.
If this is true, then the website is giving incorrect advice, and to me it penalizes genuine mistakes.and honesty. So you would be better off NOT informing the conductor and trying to check-in at the next stop.