An OV-chipcard can be used for all means of public transport, so not only for travel by train, but also for travel by bus, tram and metro. Any season tickets you may have loaded onto the card might only entitle you to travel with discount with some of them, though. Most notably NS Flex season tickets; these are only valid for train travel and you will always be charged the full fare for any journeys made by bus, tram and metro.