Jan 19, 2016 - 9:57 AM
in reply to charlotte5
The link under "More on other websites" takes you to the rhb.ch site. Is that the one you're struggling with? It indeed looks a bit complex, but I just tried it and all seemed to work fine. As you're entering your data you will also be asked for reductions. That's where you select the Swiss Travel Pass, so you will only pay the reservation fee of CHF 10 per person.
On the way back from Tirano to St. Moritz you can take any local train without reservations. That's what I would do, in order to be flexible. But if you want to book seats for that leg too, you can.