The most straightforward option by train would be Amsterdam-Hamburg-Copenhagen-Göteborg-Oslo. It easily takes two travel days to get to Oslo.
From Oslo you can go easily towards Bergen or Trondheim. If you take the Bergen line, make sure to at least take the train once by daytime, views are stunning.
Geirangerfjorden is a bit more challenging to get. Either you take a bus from Bergen to Ålesund, from where there will be busses to the Geiranger.
From there you can navigate by local buses again to the nearest railway line (Åndalsnes - Dombås) to get you on track to Trondheim.
Or you go all the way back from Bergen to Oslo, and get to Trondheim from there. For Geiranger you need to take buses anyway.
Nighttrains exist in Norway, and are the most comfortable ones in Europe. No Nighttrains from Norway down south to Europe. Plenty of ferries though, also overnight. With Interrail you get a discount on some of them.
A week seems a bit tight though, although you can save some time by taking overnight ferries from Copenhagen to Oslo, or somewhere from the north of Germany /Denmark. But not very familiar with those.
Check your routes on www.entur.no, although summer schedules won't be up yet.