Yes the one-country is fully valid on Domodossola - Locarno, like all Swiss tickets.

There's a map available : https://www.interrail.eu/content/dam/miscellaneous/Map%20of%20Switzerland_SBB.JPG

Thank you, that’s good to know! Thanks for the map - it looks like Domodossola - Brig is also included so can I assume it’s OK to make the journey back into Switzerland on that line using a Switzerland-only pass? I’m slightly thrown because technically I’ll be starting a journey in Italy but I can see the logic of the link on the map. 

The Centovalli Bahn is a very nice route.

Yes it serves as a link between Valais and Tessin, quite useful ! Domodossola is actually the border point and the railway to Brig is fully operated by SBB and BLS.

Brilliant, thank you very much for your help!

Thanks for that map link! I see the bus route from Tirano to Locarno is marked green, so also included in Interrail? I’d possibly need to take that… do you know if we can just hop on the bus or are reservations required? 

EDIT - Just found the answer elsewhere! 

On the Bernina Express Bus between Tirano and Locarno you need a reservation.