Goeiendag, bonjour!
Did that already several times from Brussels.
Norway does not have a very extensive rail network, but it works quite well. Most mainlines between big cities only have 3-4 connections a day. Don’t forget that Norway has an insanely difficult geography and is sparsely populated. It’s absolutely not that frequent as in Belgium, only the region around Oslo has good local services comparable to what you’re used too.
The railway network in Norway like a star, and very centred toward Oslo. So avoiding Oslo, will be difficult if you take only trains 
. All (normal) trains are included, for the regional trains you’ll need reservations.
Which you can get by calling EnTur (see below).
Sometimes busses are recommendend, to shorten your route. E.g. to get somewhere like from Ålesund to Bergen or Bergen to Stavanger. Going over Oslo for those, is time consuming. Whatever the options, you’ll get amazing views on the way. Buses are unofrtunately not included in the pass. But directly west from the North, you can only do by ship (Hurtigruten) or Plane (but then you’ll miss all the amazing landscapes);
For Norwegian trips, best plan here: https://entur.no/
There’s an interesting ferry connection to Norway to from Eemshaven (Nl), Hirsthals (DK) and Copenhagen (DK) should you want to return via a different option to Belgium.
There are not many connections from/to Norway. The ones you mentioned and also there would be one from Östersund via Storlien to Trondheim. But not sure if this connection is running right now as they are electrifying this line.
In general except the line from Myrdal to Flam all important lines should be included with Interrail in Norway. But for long-distance trains you need a reservation nowadays.
The railway line from Bodo to Trondheim and also from Bergen to Oslo are really scenic. To Narvik only the last part is interesting IMO.
Also Kristiansand-Stavanger is nice in my opinion.
You could also think about using partly a ferry on the way back (but then you have to pay extra). E.g. from Kristiansand to Hirtshhals.
Thanks to both of you for the answer. I’ll look at those idea more in detail.
I will look option by bus and/or ships for sure at some point.
Delphine
Precise the reverse was also my 1st trip going there=pre 2000. It is BUS from FAUSke (not Bodoe=that means 60 km come back same way, and prices in NO are sky-high) to Narvik, connects with train.
Think agian if you really want to go that far north-Narvik is in fact just a boring big port for iron-ore.Bodoe is also not much more as a port for ferries and buses. The sights from the train are nice and unusual for Vlamingen, but not that spectaculair-plus that often the only train runs in the night-but night is very, very short around 21/6 over there.
You can cut off using the mid-height line Sundsvall-Óstersund-Trondheim.
IF you have enough time you could also think of the Swedish INLANDSbanan that runs north from Östersund to Gällivare=on the Boden-Narvik line, only in summer time. All organised around touristy things- I did this when it still was a normal train with dieselsets.
