Zurich to Betten on Xmas day

Hello David, and welcome to the forum! It's correct that seats can be reserved for the first part of this train trip, but not for the last part. This is because an InterCity train runs between Zurich and Brig. Seats can be reserved here, but it't not obligatory. A smaller regional train takes care of the transportation between Brig and Betten. Seat reservation isn't possible for such smaller, local trains. This can be checked in the Swiss timetable. Connections for which seat reservation is possible are indicated by an 'R', connections for which seat reservation is obligatory are indicated with an 'R' in a rectangle. Please refer to the Swiss Federal Railways (SBB) website for further explanation. I'm not sure whether the train between Brig and Betten will be that full. Even in summer high season it's quite doable, but I don't have experience with this route on Christmas day.

You can arrange for your seat reservation through RailEurope or through the Swiss timetable. Enter your connection, fill out the right date, click 'Search' and pick your preferred connection. Then click 'Fare/Buy' and follow the instructions in the ordering process.

Good luck with your further preparations, and have a great stay in Switzerland!