About tickets and reservations
The Wilhelm Tell Express offers luxurious traveling and is therefore 1st class only. You need a 1st class ticket or a 1st class rail pass. On top of that, it is required to pay a surcharge. You can best book the surcharge well in advance because it includes seat reservations and can be sold out. The supplement also gets you a souvenir, a meal in the ship restaurant and a travel guide book.
If you have a 2nd class rail pass, you can buy an upgrade ticket to 1st class.
You're not required to travel the full stretch from Lucerne to Locarno/Lugano or vice versa. The surcharge is fixed, and needs to be paid for each trip and traveler, regardless whether you complete the trip or not. We do recommend to travel all the way though!
www.sbb.ch: Wilhelm Tell Express timetable and surcharges