Buy train tickets in advance in Interlaken & Bern?

I am taking a train from Interlaken-West to Bern and then from Bern to Zurich. I know which exact times and trains I need, so I thought I might as well book in advance, but the ticket must be mailed to me and I don't have the time. Will I be able to buy the tickets at the Interlaken-West and Bern train station, or is it necessary to book in advance?

I feel silly because I think it is perhaps obvious that a train station would sell tickets, but I just need confirmation to feel settled.

Hi katevalk,

You can buy the tickets at any station.