Jun 19, 2015 - 11:50 AM
Hi tweetyr78, thanks for joining MySwissAlps!
You're right, most trips within Switzerland don't require seat reservations. The Centovalli line between Domodossola and Locarno is run by smaller regional trains and there may be somewhat limited capacity on peak moments during high season, but I don't think you should worry when traveling with 5 people. Moreover, it's not even possible to make seat reservations for this leg of the trip.
Note that you'll be traveling abroad first, when getting from Geneva to Domodossola. The direct EuroCity trains do require seat reservations. You can best enter your entire trip in the timetable: enter Geneva as your departure station, Locarno as your arrival station and Domodossola as your via-station. The timetable will display all possible connections. Click on any of them for more details. For each leg of the route, it will be indicated whether seat reservations are required or possible.
I hope this helps you out!