Plane is expensive to get there and the drive through "Parc des Laurentides" is amazing (although after 100 times, it get tiring)
I took the train to go there once and it was a fantastic experience, we stopped in every single remove village on the way. Those village were often only accessible by the train (not by car or boat). I don't know if the train still exist though
Car are very cheap to rent from Montreal or Quebec city
BY AUTOMOBILE or tour bus are the best from Three Rivers or Quebec City.
AUTOMOBILE a must or bicycle if you can cover great distances. There are bike paths all over. There is one all around Lac St. Jean.