There is already a discount card in NL. Costs €55 a year, then entitles you (AND THREEE PEOPLE TRAVELLING WITH YOU!) to a 40% discount on all trains after 9am, and all day at weekends. It's a no brainer, 2 or 3 train trips a year and it pays for itself. It's a very simple way of implementing an off peak discount.
http://www.ns.nl/cs/Satellite/travel...=language%3Den