Depends whereabouts in Northern France you're going, but in Nord Pas de Calais there are some TER-GV services which are operated by TGV trains in their spare time. As Polarbear says, there are also routes with bus services instead of trains as part of the timetable - look out for services described as 'autocar'!
Yes, do watch out for that! If my memory serves me correct, all services operated by TER are listed on the timetable, be it a bus, coach or train service as it's all under one umbrella. You may want to get in a 'TER intercités' service for a break from DMUs - LHCS, some coaches have compartments.
I don't know how well travelled you are on French railways, but we always used to make two mistakes in some our earlier rail journies when in France
1) We didn't 'composter' or stamp our ticket on the machines at the platform entrance before boarding the train
2) Unless you only plan on going the one-way, you need a return ticket - 'un billet aller-retour'.