Region: Western and Northwestern Switzerland
SBB from Bern to Interlaken Ost
(Direct ICE connections between Germany and Interlaken)

Region: Central and Eastern Switzerland
SBB from Lucerne to Interlaken Ost 

Region: Italy / Valais
SBB from Domodossola to Brig,
possible change to regional train (BLS) to Spiez,
change to SBB to Interlaken Ost. 

Interlaken Ost train station
Head toward the town centre, walk through the English garden (two minutes), 
cross the Beau Rivage bridge on the right, then keep right on the footpath along the river.

Online train timetable
www.sbb.ch