For trains departing Sweden, use
http://www.sj.se/ - you can easily get e-tickets direct from them, and it's clear which class you're booking.
On the Stockholm-Oslo route, there are two classes of service. First class on the X2000 service looks like this:
Between Copenhagen and Stockholm, also book via sj.se - same train. There's only a few direct services, but there are more if you change in Malmo (as well as a very comfortable overnight service from Malmo).
Between Copenhagen and Oslo there's no easy rail route, but there is a direct, overnight ferry service which is very convenient, via
https://www.dfdsseaways.dk/
For Norway in a Nutshell, see
https://www.norwaynutshell.com/
If you need just the train from Bergen back to Oslo or v/v then
http://www.nsb.no/