are the train rides from Milan to Interlaken ost nice ? It’s a long time so hope so..
EDIT: I've done this in the direction from Interlaken Ost to Milano Centrale but it works exactly the same in the opposite direction, although I much prefer the Bernina Express southbound (Landwasser Viaduct).
So does the time it will take.
Do you want fastest or most scenic?
Most scenic is, by degrees, slowest.
Fastest is least scenic.
Depends which you you choose.
Most scenic of all - any train westbound from Interlaken Ost to Spiez, a crossroads station on the main north/south rail line which the Interlaken line branches off. Almost all trains heading west (and there are a lot) will call at Spiez, from the lowliest all-stops to InterCity Express (ICE) trains going to Germany and everything in-between. Change to a Lötschberg Express to Brig. A big lesson to learn about Swiss trains is that "Express" means slow and scenic. Yeah, I know. That train takes you way up into the mountains around Kandersteg (steep but no cog wheel, it is all adhesion) and along the cliff face where you can look down into the valleys and has one shortish tunnel and a few very short. If you take the main line trains via Visp you spend most of your time in a deep tunnel - no scenery. That why for most scenic you need the Lötschberg Express. Get off the train at Brig (it usually terminates there but not always) and go upstairs and outside to the street, walk a block or so and catch the Glacier Express in the middle of the street. The little red trains go the same places at the same speed with the same scenery as the named expensive tourist only trains. Many hours later and after having seen magnificent views and glaciers and lakes you will arrive in either Chur or St Moritz. Preferably after an overnight stop, but sometimes doable in a day - ask if this interests you - change to the Bernina Express (same advice about regular trains) for the jaw dropping, even more spectacular ride to Italy. Up and over the most incredible scenery. After an outside corkscrew loop where you can see the front of the train going under the back of the train, you are in Italy and the train ends at Tirano. Cross the plaza and get on a TreNord (a subsidiary of Trenitalia) train with a destination of Milano Centrale. That train follows Lake Como all the way down as far as Lecco, and then goes inland. If you want to stop on this route at Lake Como find a place to stay at Varenna which also has a ferry over to Bellagio (but that is a whole 'nuther adventure).
Too long? Less off the scale fabulous but still most excellent and somewhat faster by several hours - is to take the eastbound hourly narrow gauge ZentralBahn (part of the Golden Pass route) via Meiringen (Reichenbach Falls of Sherlock Holmes infamy is a short walk from the station and has a funicular going up the falls), the cogwheel ride over the Brünig Pass to Luzern (Lucerne). Change to a train to Chur and continue as above.
Still too long? When you get to Luzern change to the fast train to Milano Centrale (Milan Central) station. The train will go through a very long and deep tunnel under the Alps.
Fast and not particularly scenic - west from Interlaken Ost to Spiez, change to a direct train to Milano Centrale which will go in pretty much a straight line through very long and very deep tunnels to be the fastest route south. The only way you could have a longer tunnel is to go to Zurich and change to a train to Milano Centrale because this train uses the incredibly long Gotthard Basistunnel. But these are fast routes.
Where do you think you would like to put the balance on the time vs scenery seesaw?