Tu use the scenic route and the old tunnel, make sure to take train which starts Erstfeld and termitates at Bellinzona or the other way round. Erstfeld is key. If you start at Arth-Goldau you might end up on the Eurocity train to Venice St. Lucy (Venezia Santa Lucia).
The line is operated by the Swiss Federal Railways (SBB CFF FFS).
I did this excursion from Zug, where my sister lives.