I tried the sub.ch site and can't get it to let me choose the specific itinerary but maybe it is because I am still too early, as trying to book for August 5th. SNCF had same problem, said not available yet to book the 6:51 departure out of strasbourg, because at least then I had it down to just 2 tickets if did the strasbourg to zurich one via basel and then zurich to vienna.