Hallo Peraurroa94
A specific connection in Switzerland can never ‘sell out’ regarding ticket sales, because
- Regular tickets are always valid for either a few hours or for a full day, not for a specific connection (excepted supersaver tickets, but those are not available anymore on the day of travel in 2nd class)
- A major part of the passengers does not buy single tickets for a specific connection, but holds some day, monthly or yearly travel pass allowing unlimited travel
Therefore it is impossible to limit the passenger numbers on a specific connection via tickets sales. The latter works only via additional mandatory seat reservations on a few specific tourist trains.
In many cases (railway stations, bus/tramway stops in major cities), tickets could also be purchased from vending machines.