As per previous advice from Arno, we will be travelling from Tirano to St Moritz on Tuesday. Can I buy tickets on the day at Tirano station? Or should I buy beforehand?
You can buy tickets in advance or on the spot. Prices should be about the same. However, if you travel with the Bernina Express instead of one of the regular trains, you also need a seat reservation. You'd better buy these as soon as possible as seats can book out. You can buy them along with rail passes or tickets if you need those for your holidays too. Please see www.myswissalps.com/ berninaexpress/ tickets and www.myswissalps.com/ train/reservations/ price.
Enjoy the trip!