There are no scenic trains starting in Zürich but trains run very frequently in Switzerland so there is no problem getting from Zürich to the start of the different scenic routes. All routes are possible to travel without reservations.

Please ask again if you have more questions after looking at the link below. 

https://www.swissactivities.com/en-ch/panoramic-trains-in-switzerland/

Are you thinking of day return trips from Zurich on scenic routes? In my opinion the most scenic routes are Chur to St Moritz and on to Tirano. Also Chur to Brig but these cannot be done in a day from Zurich. Other good rides are Bern to Brig and Arth Goldau to Bellinzona. Use the SBB app for detailed timings. Don’t waste money on the Glacier Express unless you want to dine in a restaurant car (expensive but superb). This train has a very high reservation fee which is obligatory. There are lots of local trains on the route of this train.

You can travel the same route as the Glacier Express but with regional trains without reservations. When you look for the trains from Chur to Zermatt you need to enter a Via such as Andenmatt to get the correct route and not the fastest route.

Nearly all routes in Swiss have scenic parts. In the general site-country info, is a list of specific lines. In fact even some lines going out from Züri-along the lake, are quite scenic (IMHO), and you can even also use other nonSBB lines-see again main list in info.

There are 5 or 6 lines that are heavily promoted as such-mostly becse they also have touristy=big supplmt special cars on them (though often only in main season, this will cease shortly), like Panorama Express Gotthard, Furka-Oberalp, Roofs of Swiss, Golden LIne (MOB to MOntreux), Bernina and Albina (Graubünden-so-east part toward Italy). ALL these lines you can enjoy for ´free´=no extra pay in the hourly normal trains that also run over them. With some planning you can do most of them also as daytours from Züri (a very expensive city) and come back same day.

The main exception=pay a lot extra, is the BOB=Berner Oberland, which runs south from Interlaken to that Jungfrau mountain (highest line in EUR) with also some sidelines.

Yes thinking of a day trip 

Which one do you recommend perhaps for half a day trip from Zurich

Zurich is a long way in time for the best routes if you only have a half a day. This itinerary gives a trip over the Gotthard Pass which is very scenic and has several spiral sections and you come back through the base tunnel -the longest tunnel in the world. 

Zurich 07:05

Arth Goldau 07:45  dep 07:54 (Mountainous section from Arth Goldau)

Bellinzona 10:00 dep 10:47 (57km tunnel)

Zurich 12:27

None of these trains need reservations.

From Zurich also possible taking a IC/EC train to Lugano (via Gotthard Base Tunnel or if you have time via Gotthard line with IR trains) and from there the Bernina Express Bus (compulsory reservation) to Tirano and from there local trains to St.Moritz and Chur or direct services of Bernina Express (compulsory reservation except on first carriage in the front of the train) to Chur. Chur-Zürich with IC/ICE. 

Auch schön: Voralpenexpress zwischen Luzern und Pfäffikon. Z.B. Zürich HB ab: 9.35 Luzern an 10.25. Luzern ab 10.39 Pfäffikon an 11.54 Pfäffikon ab 12.08 Zürich HB an 12.42