Hi bat23,
I have a couple questions first:
Are you taking a cruise on a large cruiseliner or are you referring to the ferry/boats that make the trip in a few hours?

Flam is reachable by train but how do you plan to get to Kaupanger, it is quite a ways away? If you have a car then you can take the ferry from Kaupanger to Gudvangen (as we did) and then exit the ferry with your car at Gudvangen.

What portion of your trip will you have a car?

If you are taking the train from Oslo to Bergen or visa versa, then you can take the ferry from Flam over to Gudvangen, catch the bus and then continue on the train to your destination.

This website might help you a bit more with the planning:
http://www.fjord1.no/eng/boat/boat-timetables/tourist-services

If you have a car you can drive parallel to the Flam railway and see everything it lets you see IME - I took the train but noticed the road right along the whole way it seemed.

Is it worth it - I am a rail buff and have traveled European trains for decades and seen just about every ballyhooed scenic train ride - I would not rate this anywhere near the top but it is very scenic - it features a stop and photo op at a thundering waterfall en route - the ride is fairly short.

I took the Gudvagen-Flam boat ride and IMO nothing can top that - anymore may have been too long - can see whales and sheer cliffs - outstanding scenery on Norway's largest fjord. Not sure about the other boat destinations - but the classic Norway in a Nutshell thing is from Gudvagen to Flam or v.v.

For details on Norway in a Nutshell components, all of which are by public transit that you can pay for as you go along if not getting the all-inclusive ticket check out these sites - www.ricksteves.com; http://www.budgeteuropetravel.com/http://www.norwaynutshell.com/.

we did Flam to Gudvangen and I have to agree with pal Pal...outstanding. The train ride from Myrdal to Flam is as Pal describes...we enjoyed it we did it as part of the N-I-N, left Oslo at 8:00 am, via Myrdal, Flam, Gudvangen, then bus to Voss and train to Bergen...arriving at 6:00 pm. The prices have skyrocekted since then, but it was certainly memorable.

Bat23:
Touristy, tourist trap, etc...well hell yes...most every "touristy" site is worth seeing and experiencing at least once...that's why tourists flock to them. Once and for all, I wish each and every Fodorite would bite the bullet and admit we are all "tourists", mere foreigners in a foreign land. Try this cleansing...it won't hurt but for a moment or two.

If you have a car you can drive parallel to the Flam railway and see everything it lets you see IME - I took the train but noticed the road right along the whole way it seemed.

But it is a dirt road until the valley floor.

Bat23: You might want to take a look at my trip report from 2009; click on my name to find it. We did not regret the NIN trip but as I mention in the report, because we subsequently drove through the fjord area by car, in retrospect it was unnecessary.