AlbertoSA
Any kind of ‘ticket’ is never a ‘booking confirmation’, just an ‘admission fee’. For securing seats, an additional seat reservation is required. But this is possible only on EC/IC trains (so only between Interlaken and Brig in your example) and a few ‘panoramic’ tourist trains as mentioned in the blog article Danhiel linked above.
Note that we resident Swiss travellers never reserve a seat and therefore are not even used to check whether a seat is reserved before sitting down in an IC…
Because even if a train is ‘full’, there are usually still a few free seats, especially towards both ends of a train. Another hint is to avoid peak times, e.g. avoiding the majority of commuters by taking the earliest services or departing after approx. 08:00 am.
But there are not too many commuters between Grindelwald and Chur anyway. 🙂