Assen’s main station connects it to the Zwolle-Groningen train line. Within 25 minutes you can be in in the city of Groningen. At the main station you can also take local and regional busses to travel within and around Assen or rent a bicycle for a small fee.
Public transport in the Netherlands is provided for by multiple private companies. To plan your traveling by train, we recommend the website of Nederlandse Spoorwegen. For visiting smaller places, traveling by bus might suit better. In this case, we advise the website of QBuzz to plan your trip, they provide the bus connections within the north of the Netherlands. If you need to combine traveling by bus and train, the website of 9292OV will be an more easy tool to plan both at once. Note that 9292OV is a third party and does not provide public transport itself.