Sep 5, 2016 - 4:56 PM
in reply to Mirike123
No worries, there are other ways to reach Pontresina as well. Remember that most of the trains and buses in Switzerland are simply public transport and people travel spontaneously without any advanced planning or booking. This particular bus happens to be on a scenic route for tourists, and it only operates once per day, hence the need to book seats. These routes are popular so booking late is risky, but people may cancel.
You can visit any staffed station as soon as you reach Switzerland and see whether they have seats available. If not, you can travel via Milan. You will have to pay for the Chiasso to Tirano leg, but tickets start at only € 14, so that's not much different from the bus reservation costs. From there you can take a train across the Bernina pass to Pontresina for free with your Swiss Travel Pass.
Another option is to travel via Göschenen - Andermatt - Chur, which includes part of the Glacier Express route and the northern section of the Bernina Express route. That might duplicate with your other travel plans though. Other options include buses from Bellinzona to Thusis and the bus from Lugano to St. Moritz (Palm Express). These are free too, but again you would need a reservation. Anyway, there are many options, just check the timetable. Since you almost depart I would simply choose while in Switzerland and arrange for seat reservations, if needed, there.