Apr 13, 2015 - 5:52 AM
You can book this train via this page of Rail Europe US, but you have to book it in a special way because they don't offer Swiss Half Fare Card discounts yet: search for trains from 10 AM from Domodossola to Milan, and do not select a rail pass. That should result in the same train, which leaves Domodossola at 10:17 AM and arrives in Milan at 11:37 AM. For the leg from Zermatt to Domodossola you can simply purchase a 50% discounted ticket in Zermatt. You will board the train to Milan in Brig. The worst that could happen is that you don't have a seat for the first 25 minutes from Brig to Domodossola, because your ticket including the reservation is valid from there onwards. Of course you can also book from Brig to Milan to prevent that, and buy a discounted ticket from Zermatt to Brig in Zermatt.
Alternatively, you could book from Brig (or even Zermatt) to Milan through the SBB timetable, but they don't offer discounts for the one month Swiss Half Fare Card. In case of Italy though, the 1/2/3 year option should give you the same discount.