On to the wiki page of Stazione di Pré-Saint-Didier is stated (under “Movimento”)
La stazione era in ultimo servita da treni regionali di Trenitalia
fino al 24 dicembre 2015, giorno in cui la linea è stata sospesa al
traffico per decisione della Regione Valle d’Aosta e sostituito da
autocorse.
Google translates this as
The station was finally served by regional trains Trenitalia until December 24, 2015, the day when the line was suspended traffic for decision of the Valle d’Aosta region and replaced by bus rides.
I’m afraid you’ll have to take the bus.
Credit:stackoverflow.com‘