Beste antwoord door Slidestops
Tickets via Stena used to be valid to all stations in the Netherlands. This changed however.
Trains on Gouda - Alphen are not branded NS, but are operated by NS. So I would see no problem in using it over there.
Our information about this tickets says: Not valid in trains of: Arriva, Breng, Connexxion, Blauwnet, Keolis Nederland or any other Bus/Tram/Metro company. R-Net is not named in this list.
As our own information about the TAG-line says: All subscriptions and tickets valid for NS, are valid for Gouda - Alphen.