In Switzerland you don’t need reservations, only on Panoramic trains like Glacier Express. 

Wich route are you trying to reserve?

Waste of money to reserve seats in Switzerland… except if you use the touristic trains.

You can travel on the hourly local trains over the route of the Glacier Express. On the Bernina Express route you can travel in the ‘ordinary’ coaches. My experience was that the trains had large windows, some of which opened. One train had a limited number of seats that were not reserved for parties - the ‘Golden Pass’ train from Montreux.

If you share your travel plans you will get advice. 

Hi, I would like to be sure to reach airport in Zurich in time, so I would like to have a seat reservation. Is this not necessary at all? What about high season, are trains full of people and luggage? We are travelling in July this year. Any experiences of this will be helpfull

Reservations in Switzerland are a waste of money. They're not necessary at all.

And they're actually impossible to make on most trains. Simply go to the end of the trains for more seats, people are lazy and enter in the middle

Trains in Switzerland run frequently and have loads of capacity, you can check occupation on SBB app (a very useful app !).

There are more than a dozen trains per hour between Zurich HB and Zurich Airport.