There are two different rail services available when taking a journey by train from Zurich to Bellinzona.
No matter which train you take, you will be travelling on a lovely route, which will take you by four lakes; the Zurichsee, the Zugersee, the Lauerzersee and the Luzernsee.
Though south of Fluelen, on the shore of the Luzernsee, these two train services take different routes:
(1) The 'Traverso' (IR) trains still take the older 'classic' and much more spectacular route via Goschenen.
If you are new to Swiss rail travel, have the time available and the weather is behaving itself, taking these trains is the way to go.
(2) The express IC and EC trains are faster, because they make use of the world's longest rail tunnel, the Gotthard Base Tunnel, which by-passes the most beautiful section of that 'classic' route.
So these trains will get to Bellinzona quicker, but the journey won't be as wonderful!