Seconding what Annika has said, it is extremely unusual to find a Swiss train with no seats available.
I travel both first and second class, using a half-fare card for discounts. I tend to use first class for the longer trips. Second class is perfectly comfortable and acceptable. First class is less full, and the seats may recline just a little bit , for example
I've been visiting Switzerland since 1980, and have made over 80 trips there.
In first class, which I tend to use for longer trips, I cannot recall a time when I could not get a seat.
In second class,maybe once frorm Bern to Burgdorf during the evening rush hour did I choose to stand instead of walking through the train to find a seat. That train would likely have been full or near full most of the way to Zürich; commuters were going home after work. Although I used second class for that very short ride, I passed through first class cars and there were empty seats.
The cars nearest the restaurant cars on the mainline trains tend to be fuller. Also, if you are going to a station which the regular riders know to be a terminal station (tracks dead end at the railhead) , such as Zürich for main line trains, the cars that are nearest the locomotive, which are often first class depending on the train consist ( car/class makeup) will fill up first. Knowledgeable Swiss travelers my try to sit in those cars.
At stations which are not terminals, such as Bern, the cars at each end are further from the central pedestrian walkway.
These are not absolute rules. The newer IC trains are made of train sets with a particular configuration, and two of them may be coupled together to make up a longer train. Thus, first and second class are distributed throughout the train in those cases.
On boats, the first class sections have better seating areas, but the second class are fine. First class might have the highest outdoor deck, for example.