I've done 3 trips. First was Oslo to Bergen including the Flam line and also a stop at Finse just between trains. This is the highest station on the line and in winter the village is only accessible by train, no roads. Even in June there was a lot of snow however the hotel there is closed in June, it's their low season between skiing and walking. I stayed at the hotel in Voss that is accessed directly from the station platform, Flesches I think it's called. Beautiful historic wooden building. I had a room facing the lake as this avoids noise from the trains. Loads of hotels in Bergen. I had no trouble with the trains, but do book especially for Flam as the trains fully sell out days in advance.
Trip two I did Oslo to Bodo stopping over in Andalsnes at hotel Belle Vue just up from the station. Onto Trondheim I stayed there though the hotel there was not great so I won't suggest anywhere there. It was wonderful with a lot of snow, crossing the Arctic circle by train too. I stayed at Clarion Hotel Gran about 10 mins walk from the station.
Last trip I did Oslo to Stavanger stopping over in Kristiansand. There is some wonderful beaches around there if you go in Summer. Onto Stavanger I enjoyed it, with a trip on the Fjords and a walk to the Kervaig boulder. Wonderful trip.
I have photos of all these I'll add links if of interest when I get home. The first two routes to Bergen and Bodo were the most scenic from a railway point of view, I'd agree with others that the route to Stavanger is less scenic, but still very nice.
The upgrade to Komfort class cost a flat fee of what worked out at about £10 last time I was there, regardless of journey length. Free unlimited drinks and more legroom was worth the extra for the longer legs, especially to Bodo.
I absolutely love Norway. I had two trips there cancelled last year due to Government restrictions. Can't wait to go back.
Worth noting most cities are connected by reasonably frequent flights which can be fairly cheap if you don't want to train both ways. Oslo, Bergen, Stavanger and Trondheim all used to have direct flights to London Gatwick too. Whether and when these come back is anyone's guess.