Hello Ruebouwoak69
The Bernina Express is not operated by SBB.
You can book your seat reservation on the operator’s website, the Rhaetian Railways:
https://tickets.rhb.ch/en/
Keep in mind, the Bernina Express is not running between Pontresina and Tirano for the period 19 November to 25 November 2023 due to engineering works.
Kind regards,
Roland