Probably bus, although you can sometimes find special fares for trains. Check prices and look for schedules at www.eurolines.com
Look for special fares on trains at the Belgian, German and Swiss national rail company websites. The links are here: http://www.eurotrip….

You might find a cheap flight that’s as cheap as the bus. Use www.whichbudget.com to see which low fare airlines fly which routes and check our “cheap flights” forum for more search engines, links and tips.