This is the part I'm not clear on:

> Children under age 16 travel free of charge when accompanied by at least one parent holding a Swiss Travel System ticket

What exactly is a "Swiss Travel System ticket"? Is it just a Swiss Pass/Swiss Card or does it also include a plain ticket? Also are the mountain trains, Jungfraujoch in particular, part of the "Swiss Travel System"? Since the Swiss Pass doesn't cover it, I'm thinking perhaps not.