SWISS is working together with SBB to make it possible to book a flight + train combo to an increasing number of Swiss destinations.
Because of this you can now book directly to Bern, Interlaken, Luzern, Lugano, Lausanne, Geneve, Bellinzona and Lugano. And I expect the number of destinations to increase.
It is a good deal, as for example I just booked Brussels - Interlaken, and it was only 9 euros more expensive than Brussels Zurich would have been. AFAIK the offer is available with other airlines as well, as those trains have been added as flights in the GDS system.
Even though the ticket you get will mention a particular train the tickets are in fact flexible, and you can take any train on the route. You could even combine this with another ticket. eg Book to Bellinzona, and buy a Bellinzona - Locarno ticket if that is where your real destination is.
More here: https://www.swiss.com/ch/en/book-and-manage/partner-offers/swiss-airrail