The real question is how are you going to get from Oslo to Bergen without it? Doing the NiN RT from Bergen is only a little shorter. You don't want to skip Oslo. And it makes little sense to FLY from Oslo to Bergen and then turn around and do the NiN from Bergen.
So I don't really see the point in trying to omit it. We thought the "lunar" landscape portion was very interesting, but the rest was ordinary. Remember, the real NiN experience is Oslo, Bergen, and in, out , and through the fjord. The train is mostly just the means of transport between these three points. Any scenery from the train is just a bonus.