Any European country offers train connections to Switzerland.
Within Switzerland, Intercity trains can take you to Interlaken Ost. From Basel, there are direct trains to Interlaken Ost. From Zurich or Geneva, one must change trains in Bern.
From Interlaken Ost, Lauterbrunnen can be reached in 20 minutes with local trains. From Lauterbrunnen, there is a postal bus that can take you to Isenfluh in 14 minutes.
Interlaken can be reached from Basel/Zurich/Geneva via Bern or Luzern/Brünig Pass.
From Interlaken, follow the road signs to Lauterbrunnen. After approximately 2 km after Zweilütschinen, turn right at the sign Isenfluh and drive to a winding route to Isenfluh.