It's a nice idea but not entirely ready for prime time, since it only covers WA-only itineraries. The WSDOT folks on social media have been fielding tons of questions from people complaining that they can't use it to travel between SEA-PDX or SEA-VAC, which are obviously two very popular segments on the Cascades.
Their solution? Book separate tickets for the same train. In other words, if your 17-year-old wants to travel from Seattle to Portland (on their own or with their family), they'd have to book a ticket from Seattle to VanWa, and then a separate ticket from VanWa to Portland. Because Amtrak's reservations system isn't sophisticated enough, this has to be done with two completely separate transactions, as opposed to a multi-city ticket.
Understandably, segments involving Canada are a different beast. But it seems as though with a bit more upfront effort, they could have figured out how to cover the entire SEA-PDX segment under this promotion. Instead, they are asking people to navigate Amtrak's complicated user interface and also blaming the state of Oregon for not stepping up. Not a great look, if you ask me.