On-line Train Reservations for Swiss trains?

I don't know this for sure, but I think I have heard that you can't reserve trains of Swiss Rail. Anybody?

What trains are you trying to reserve? Very few trains in Switzerland require reservations, and for others they are unnecessary or not even possible. If you are concerned about finding seats on a train that does accept reservations, check the SBB site a few days ahead; it will show how crowded the train is at that point. You can always book seats at the station then if it looks like the train might be too crowded. If you are trying to reserve a seat on one of the scenic trains such as Bernina Express or Glacier Express, you do that on the designated train's own website. For those two, it is the Rhätische Bahn website: http://www.rhb.ch/index.php?id=1643?&L=4

I'm specifically trying to reserve two seats on a train from Interlaken to Innsbruck at a certain time and day in Sept. When I go on the SBB site, I seem only to be able to reserve in conjunction with buying a ticket (I already have a Swiss Pass). I haven't found how to separate the two actions. The advice about making reservation a few days in advance if the train looks crowded is appreciated and may be my best option.