For a lousy €12 (including beverages and hot lunch) we  made a 12 hour trip from Bangkok to Chiang Mai. The train (a south Korean Daewoo) and the tracks were in good condition so we had a smooth ride.
However nice the landscape (endless rice fields and forests) is, it was a very (to) long trip. So on our way back we will take the airplane from Thai airways. That will cost us twice as much: € 24... :-)