I've discovered that SNCB sells on their website cheaper Roosendaal-ticket and it's possible to buy from Roosendaal to any Belgian station, the price is cheaper than NRT (for example 6,1 EUR instead of 8,6 EUR of NRT ticket for Roosendaal - Antwerp.
Yep, NMBS/SNCB has a special "Roosendaal Ticket(s)"
But I afraid validity term of this Roosendaal ticket may be not 15 days like NRT ticket have, but probably only 1 day, right?
Yes, you are right, domestic one-way tickets are valid for one day only
Have I missed whatever and there is better solution?
Well, you didn't mention the dates of your journeys, but during summer (23/06-29/08) NMBS/SNCB offers special ticket called "Summer Deal" (don't know why NMBS/SNCB hides details about the offer in the ENG version of the website, but in
Dutch and
French it is there). It won't save you much, but still makes sense to consider if you travel within Belgium, especially since it combines with TEC & MIVB/STIB
Other question: if it is possible to buy Roosendal-ticket on the station before departure in the Netherlands? I afraid it is not possible, because for example nsinternational.nl shows price 8,6 which is normal NRT fare.
Last time I was in Roosendaal last summer and there were no NMBS/SNCB ticket machine (and apparently it's
still not there), however many passengers were just buying ticket from the Belgium conductor on the train