Will it be possible to travel by train from Geneva to Interlaken, take in the Swiss Open Air Folk Museum at Ballenberg, and still get to Gimmelwald to spend the night?
If you get an early start, you have plenty of time. Geneva to Interlaken Ost is just over 2.5 hours away with a change in Bern. At Interlaken Ost it is another hour to the museum via Brünig-Hasliberg or Brienz via bus. You need to leave Ballenberg by 6pm to make the last connections to Gimmelwald.
You can get complete schedules at the Swiss Rail site:
www.sbb.ch/en
For Getting there enter Geneva to Ballenberg Museum,
then Ballenberg Museum to Gimmelwald.