Gonna make a post with useful tips for interrail as I discover them seeing as how the site is sorely lacking in my opinion. My pass is a 1st class one so it will be from that perspective but I assume they will be relevant for all. Will update as I go along.
Tip 1: If you have a first class pass then seat reservations are free in Norway.
Tip 2: Best ways to reserve seats IMO is to either do it directly at the train station(the most stress free way I feel like, or use this page (https://www.interrail.eu/en/plan-your-trip/reservations/how-do-i-book-my-reservations) to figure out where to call if you can’t do it directly at the station. Calling is for sure the best way if possible though. For me, whenever I take a train somewhere, while I’m at the station I just reserve my seat while I’m there, easy, and you save time. Do note, seat reservations are not always neccessary, they will always let you on the train, but if it’s a very popular route then it might be good to reserve it. For example, London - Paris, hugely popular, def reserve.