As the reservation of a wheelchair space is mandatory on international trains, you can only buy tickets for travelling abroad at staffed counters or at least 48 hours before train departure from the SBB Contact Center on 0848 44 66 88 (CHF 0.08/min.).
If you do not require a wheelchair but only need a ticket and a seat, you can also purchase and/or book your tickets for travelling abroad online at SBB.ch.