Our trip in 2024 was booked throughout using "Norway in a Nutshell", which I found very easy to use and almost infinitely flexible although possibly less useful if you're Interrailing. Our booking with them involved a train from Bergen to Voss, coach to Gudvagen, ferry to Flåm, two nights in Flåm, Flåm railway up to Myrdal and train to Oslo (on which we spent four hours talking to a Norwegian who informed us that Norwegians don't like talking to strangers!). We had luggage in two fairly large suitcases and never had any problems with them. As Killingworth suggested, maybe the fact that the railway is largely cruise-oriented means that luggage capacity just isn't an issue.