I did this journey three summers ago. It is a lovely journey, albeit long! I did both ways in daylight, which was fantastic (good July weather too)!
Havent therefore experienced the sleeper Norway-style but I imagine it would still be fun (although in the summer it might not even be much of a 'sleeper'... Both Oslo and trondelag at 1am in July was still very balmy and light!!) Maybe to the far north one day.
I would have to do some research but there are two types of train which run. One is an older loco set. The other is a newer DMU-style set. I travelled first in both sets and I would absolutely recommend the DMU diagram... for sheer space in first! It was like being sat in a lounge, with high back chairs, not fixed to the floor and big scenic windows. Thus, wouldn't be able to tell you about standard. I found a website with pictures and the diagrams on once.
Also... Hotdogs or pizza on a train?
In short... do it!!!!! It is definitely on my list of things to go back and do again one day (and not the only Norwegian thing)...
