Beware! The site norwaytrains.com is associated with a dodgy company that specializes in ripoffs. Don't take my word for it -- check the web or Tripadisor
As always, avoid 3rd party booking sites. In Norway, use the national planning site www.entur.no or the operators, www.vy.no, www.sj.no or https://go-aheadnordic.no