Jun 23, 2011 - 1:48 PM
Hello Harsha. If you'd like to purchase your tickets at the spot, that's fine. It is peak season indeed, but many trains will run up to the Jungfraujoch, and extra ones will be used if there are lots of tourists. You can also choose to buy your tickets in Grindelwald for example one day ahead, if you know for sure that weather will be fine.
As for your itinerary: it's quite a lot, but it's a great travel scheme if you want to explore as much of Switzerland as possible. Please review the day trip overviews of Graubünden
for tips on activities during your stay in St. Moritz and Zermatt, and (for example) this forum topic
on Zurich. Did you check the timetable
already? It can be of great help, as it gives exact overviews of traveling times.