Hello Judy52,Nine minutes is sufficient to make the change, but it's not much if you also need to buy tickets. Trains may wait a few minutes if connecting trains are late, but in case of extreme delays you need to take the next train to Interlaken. Pre-booking seats is not necessary for Swiss trains, unless the timetable says differently (e.g. in case of the Glacier Express). The train to Interlaken would be free if you have a Swiss Pass or Swiss Flexi Pass, so in that case you don't need any tickets at all. If you do need tickets, you can purchase them in advance, either online or at any Swiss rail station.