Hello KashiW
Your Swiss Travel Pass is valid in every train from Domodossola to Grindelwald, it means that you can stay in the Eurocity (this is also a SBB train in cooperation with Trenitalia) or even take a BLS regional train later if you want to make a break.
Please note that the Berner Oberland Pass, mentioned in the previous post, is valid only in the BLS trains.
Kind regards
Danhiel