The best way to get to Vatnahalsen is by  train form Bergen to Myrdal (about 2 hours) and then the Flåm train (2 minutes) to Vatnahalsen. It is also possible to take the train from Oslo, but this will be substantially longer. We suggest the following trains

Wednesday 1/3, Train 15.59 from Bergen , arrival. Vatnahalsen 18.07.
Sunday 5/3, Train 12:27 from Vatnahalsen, arrival. Bergen 14.57
   optionally. 15.22 – 17.57.

Buying tickets on NSB, put Bergen and Vatnahalsen into the search fields.