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 sbb-tickets.cc, 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.