It starts at Vatnahalsen, which you can get to either by walking (2-3 hours) or taking the Flåm Railway. The zipline ends at Kårdalen, next to a mountain farm called Rallarrosa. Here you may want to taste some locally produced goat cheese before heading back home. 

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If you have rented bikes, you can ride the bikes from here to Flåm. Otherwise, you can catch the train or walk back down. For more information, visit this site.