Milan~Interlaken train route?

We went through Spiez and thought it was pretty scenic, especially on the Swiss side.

The route via Spiez from Brig is the fastest, and is quite scenic. A large portion of the journey is now in tunnels however, and you can't see much when you are in tunnels. The route from Milano to Luzern is considerably more scenic as you wind through spiral tunnels and see the same church from 3 different angles and get to see the magnificent Vierwaldstaettersee (Lake Lucerne) as you go towards Luzern. From Luzern, there are 3 possible train routes to Interlaken Ost (for Lauterbrunnen). The fastest is via Zurich and Bern (and Spiez from the opposite direction) but least scenic. The second possible route is on the little BLS train through the Emmental (where the cheese comes from) which is a slow rural line from Luzern to Bern where you change to a train to Interlaken Ost. The third - and most dramatically scenic - choice from Luzern to Interlaken is via the Zentralbahn train over the Brunig Pass. Fancy trains called the Golden Pass do the route, and more frequent normal trains run by the Zentralbahn both do the same route. Full of scenery, narrow gauge, cog railway, all sorts of things to see on the route (if you wanna know what, ask). Usually from Milan to Luzern you will change just outside Luzern at Arth-Goldau because Luzern is a dead end station so the Milan - Zurich trains usually don't go into the station and you get the short run in from Arth-Goldau.

Nigel, if you are travelling from Luzern to Interlaken via Bern (or elsewhere on western areas of Switzerland), there is no need to go to Zurich. You can change trains in Olten, that transfer point for Basel-Luzen and Bern-Zurich trains. Travelling via the Zentralbahn a.k.a. Gold Pass line is the fastest option between Interlaken and Luzern anyway. It takes exact 2h.