The Bern - Lötschberg - Simplon (BLS) railway provides a service to Kandersteg from Spiez in the north and from Brig in the south.
These days, the service to Kandersteg is a "regio", stopping service, although a few trains do go through to Bern. There is also a BLS bus from Reichenbach, through Frutigen and up the Kander valley to Kandersteg. See here http://www.bls.ch/e/bahn/fahrplan.php for relevant BLS timetables. http://www.fahrplanfelder.ch/fileadmin/fap_pdf_fields/2011/330.pdf takes you to the 2011 timetable (timetables tend to change once a year in December).
-- JaneCurry - 2011-05-18