Hey Charlotte,

better to book via oebb.at 
Choose reduction “Interrail / EURAIL Globalpass”. Then you can book it for 11 euro
 

Then you can choose it:

Oh and if you want to go only to Domodossola, then there’s actually no reservation needed. Only if you go further than Domodossola.

For travel IN Swiss-and that incl. Domo, RES is optional and there is NO surcharge. SO-if you plan (why not tell all details anyway?) to ride that scenic Centovalli-to Locarno-then on, it is not needed. NOr if you out of money saving change to regional treni dalle Domo per Milano.

In fact for the trains going on to Italy (always MIlano) there is a surcharge of 11€ and that incl the seat RES. Should be possible via sbb.com-or check the overview of seewulf on howtodo-that anna fra Sverige links to all the same Questions here.

I want to go to Domodossola, then take a bus to Ornavasso. So then no seat reservation is needed? Or you still recommend it?

Also thanks for your reply!

Train might be full, but I wouldn’t buy a reservation for 11 euro if not needed… It is ridiculously expensive for a standard train.
You can see on SBB for example that it says no obligatory reservation until Domodossola.

Reservation is not possible for trips whinin Switzerland for trains to/from Italy. Domodossola is the border station.