The Treno Gottardo runs over the Gotthard route from Luzern to Locarno every two hours, and from Zurich to Locarno every two hours as well, giving an hourly service over the old route.
And it runs year long, not only in Winter. It is a nice train, and recommended if you want to travel the mountain route.
You will find it in the timetable if you search Luzern - Locarno (or Luzern anywhere south of the Gotthard) and enter “Göschenen” as a Via.