Thank you both Hektor and Thibcabe - yes that answers my question and saves a few bucks as well!

Yes, Tirano should be included. You can check on which train lines and destinations are included on the map found on this page: https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/trains-country/trains-switzerland

Thank you very much BrendanDB. Yes, Tirano is covered by Interrail, but my question is whether I need a One Country Pass or a Global Pass - since the Bernina Express journey is within two countries, (Tirano, Italy to St Moritz, Switzerland).

If you do just go from Tirano to Switzerland it's considered to be a swiss journey that is covered with an one country pass for Switzerland. 

Here's a direct link that shows what is covered with a one country pass for Switzerland: https://www.interrail.eu/content/dam/_new-structure/doc/res/See_Switzerland_by_train_2024_Geltungsbereich.pdf

The map in the link shows what's included with a One-country pass Switzerland.

Tirano is the border point, don't worry. A One-country pass is fully valid.

Other exemples: you can take the Bernina Express bus between Tirano and Lugano or even the Locarno - Domodossola - Brig route, even though these touch Italy.