If you have an 'old' Off-peak subscription and you are older than 60, you are entitled to a free RAILPLUS card. You can buy this card at a Tickets & Service counter at one of the NS train stations. You will get 15% discount on the full rate of cross-border travel.
The application period of RAILPLUS is a 10 days for paying by bank and 5 days for paying by credit card.
Please note: the RAILPLUS card does not offer discount when travelling with Thalys, Eurostar and in France.
Read more about possible discount with RAILPLUS