Brittdigian
This is the advantage of the Saver Day Pass; it allows you to freely choose your way of travel without being tied to a specific connection.
I suggest following route:
Lauterbrunnnen-Interlaken Ost
Interlaken Ost-Lucerne.
Lucerne-Chiasso.
Connections as provided by the online timetable are safe and well doable as they include enough buffer for changing trains at intermediate stations.
Interlaken Ost to Lucerne is operated by Luzern-Interlaken Express and is one of the most scenic train journeys in our country. Sit right hand side in direction of travel for the best views.