We also did exactly this. No problem. We did buy the ticket online, and picked it up in the main train station in Oslo. It wasnt cheap. It was also alot of sitting, and a long day starting smoething like rising at 5am for a 6am train. I think it would be better done over a few days with a stop or two along the route, but if this is your only stop in Norway, and you arent sure you'll be back, do it.

It looks like the kind of trip that would be nice to do over a week or more, stopping a day or two here and there along the way.