So far I have talked a lot about the major train operator in Holland NS- the Dutch Railways however there are other train operators out there that you may wish to travel with these include:
The Arriva trains operate northern secondary lines, the eastern lines and a central line. You can use the OV chip card on these trains. Loose tickets and webtickets can also be purchased if you dont have a travel card
Breng services the easterly part of the country, you can use your OV chipcard or buy tickets from the machine at the station/platform or online from NS(https://www.ns.nl/producten).
Syntus - one eastern secondary line
Veolia Transport - southern secondary lines.
Connexxion - two secondary lines
NS International in international trains, and domestic high speed service Intercity Direct
A few Dutch railway stations are serviced, even for journeys within
the country, from foreign railway operators using the NS authorization.
DB Regio, including DB Regionalbahn Westfalen and DB Euregiobahn.
NMBS/SNCB - Maastricht - Eijsden, as part of the Maastricht - Liège service
but remember to check in and out and if travelling from an NS train you
must check out of the NS train and into the Arriva train!