By default tickets are for a route, not a train. In fact you cannot even buy something from SBB that will have “train ticket” written on it.
So you can buy your ticket in advance on sbbswiss.org, but make sure you buy a normal point-to-point ticket, and not a supersaver ticket.
Such a ticket is good for any train on the route you bought it for, for a whole day, till the next day 5:00. So you can then just hop on the next train to your destination once you are out of the airport.