We took the roundtrip Flamsbana from Flam to Myrdal on a cloudy/rainy day, but even allowing for the poor weather, we did not feel it was worth the effort, and certainly NOT at the price the cruise line charged (perhaps if on could get tickets locally at last minute for less than $65 US if the weather is great, it would be worth it). That said, there is little else to do in Flam (a bus tour or two and short boat excursions), so if you don't mind spending the money and NEED something to do, OK. The train is a short ride each way, with a few stops (to allow the train coming the other way to pass at midpoint, and to see a waterfall near the top), and does have lovely views of numerous waterfalls and nice valleys and farmsteads, but at no point does one actually see the fjord. If one comes by cruise ship, the sights of the fjord and waterfalls from the ship are FAR superior to anything seen from the train. Almost a third of the ride is in tunnels, and another third the scenery seemed to be on the 'other' side of the crowded car, so unless one can sit on both sides (as in changing sides on a round trip), you will miss much of what little there is to see.
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