It's too soon, Trenitalia haven't loaded their full timetable yet so only some trains are showing.

They are currently available to the previous week but there is no fixed release schedule, you should check back periodically but there no need to worry, Italian trains don't normally sell out far in advance.

Venezia to Milano has no timetable for August yet. You need to wait for this one.

From Milano to Interlaken is never a direct train. You need to change at least once. But this timetable is already available. 

7.8. has track construction works. There's a replacement bus from Milano in the morning at 6am, 7am and 9am. Change in Domodossola (e.g. 8.48 for the first departure) and Spiez (e.g. 10.05 for the first departure) to connecting trains (later connections may be faster with additional changes).

Thanks Hektor. What is 7.8? Is it for certain I cannot get a direct train from milan to spiez on aug 7th morning? If so what would be my best alternate option? I am traveling with family and kids and we are staying close to milan centrale.

Thanks again.

7.8. is August 7th. The date you did look for.

The track is closed the whole summer. No trains.

The option is described above. Replacement bus. (Or a detour via Lugano.)

If you provide a departure time I may get you a detailed timetable. 

Possible to leave at 6am, 7am, 9am, 2pm, 5pm, 6pm.

8am is my departure but I’m flexible. Thanks much.

The first bus after 8am is at 9am.

Bus is leaving Milano Centrale at 9am. It does leave from Piazza IV Novembre near the ATM bus stop at the side of Hotel Gallia.

This is an official rail replacement bus designed for long distance travel (should be comfortable). Your railpass is valid; no additional ticket needed.

It will arrive in Domodossola at 11.20 (at the train station). Now you do have different options:

Least changes: Train at 11.58 to Spiez (does arrive 13.44; connecting train to Interlaken 14.05, arrives Interlaken Ost 14 26). Very scenic trip as this train uses the tracks over the Lötschberg mountain. 

Fastest: 11.58 from Domodossola, arrives Brig 12.33, connecting train departs 12.48, arrives Spiez 13.23, connecting train departs 13.34, arrives Interlaken Ost 13.58. Less scenic, uses a long tunnel under the mountain. 

Most interesting and scenic: 11.58 from Domodossola, scenic train ride over the Lötschberg mountain, arrives Thun 13.58. There's a connecting boat directly from the station that goes to Interlaken. It's included with a railpass. Boat departs 14.40 and arrives in Interlaken at 16.49.

Not recommended: It's possible to bypass the replacement bus by leaving at 8.43 by train from Milano to Lugano. You need to change in Lugano, Arth-Goldau and Luzern. But it's longer and has two tight 4 minutes changes of train. 

Thanks Hektor. I need to be earlier than that time in interlaken. If I can leave at 7am, can u please suggest the best route?

Thanks.

In general there are the same options as 2 hours later.

Fastest: 

7.00 from Milano Centrale (bus) / arrives 9.20 Domodossola 

9.58 from Domodossola  (train) / arrives 10.33 Brig

10.48 from Brig (train) / arrives 11.23 Spiez

11.34 from Spiez (train) / arrives 11.58 Interlaken Ost

(The other options are also available like 2 hours later.)

Appreciate your detailed help Hektor!

7.00 from Milano Centrale (bus) / arrives 9.20 Domodossola

9.58 from Domodossola to Spiez arrives 11.44am

Spiez to Interlaken OST 12.05pm arriving 12.26pm.

With family and luggages, I prefer less transfers. Is this a doable plan? Can I book the bus tickets through RailEurope? How can I exactly find the bus stand and is there only one train station in Domodossola? Anything else I should be aware of?

 My final destination is Zweilütschinen, Switzerland. Hope I can take any train from ost to there.

Thanks again for your help.

In general: I understand you are not familiar with european train travel but you are worrying too much about changing trains. Especially in Switzerland that's no problem even with big luggage. Five minutes are enough time to change and you do have a lot more here. 

Regarding the bus from Milano Centrale: As written before, it does leave from Piazza IV Novembre near the ATM bus stop at the side of Hotel Gallia (ATM is the company running public transportation in Milano). That's basically near the station itself. If you are staying near Milano Centrale ask the reception staff of your hotel about it. They may guide you if you do need help to find it.

Regarding the bus ticket: I did understand that you are holding a railpass? If that is true, you don't need a ticket. This is a rail replacement bus and your railpass is valid. You just need to add it in your planner app (it will appear by then). Your railpass is your ticket. I don't think it's possible to do a reservation for the bus (if this has been the question). It's possible to buy a ticket (you don't need one with a railpass) for 13 € but it doesn't give seat numbers, so I don't think you do need a reservation; you may ask at the station when arriving to be sure (for the train that is not running you'd need a 10 € supplement).

Regarding Domodossola: That's a small town in a mountain valley. And the bus is just existing to bring the people to the connecting train. There are no intermediate stops. It will get you directly to the station and most fellow passengers will go the same way. 

The 9.58 train from Domodossola is a RE train to Bern. No reservation needed or possible. Arrives 11.44 at Spiez (track 5). 

The connecting train is a RE to Interlaken Ost that does leave at 12.05 (track 2). No reservation needed or possible. Sit on the left hand side in direction of travel for best lake views. Arrives 12.26 at Interlaken Ost (track 5). 

Even if it's repeated: I'd always go by boat and not by train on this part. We do use it every summer trip, always with big luggage, and we are just going there for the boat. At this time of day it's a vintage steam ship. Would never skip this. But it's just a recommendation. ]

The connecting train is a Regional to Lauterbrunnen/ Grindelwald that does leave at 12.34 (track 2). No reservation needed or possible. Arrives 12.46 at Zweilütschinen (track 1).

If you search from Domodossola to Interlaken or Zweilütschinen on the Swiss Railways website, you’ll see all these trains, including maps of the station and facilities onboard each train.

www.sbb.ch  (change language at the top)

Thank you so much! If I want to take the boat, can I do that from Spiez or should I get down at Thun? I am assuming its covered by global pass too. You are right. We are first time Europe travelers and with family I want to make sure there is enough time. Thanks for your understanding.
 

If you search from Domodossola to Interlaken or Zweilütschinen on the Swiss Railways website, you’ll see all these trains, including maps of the station and facilities onboard each train.

www.sbb.ch  (change language at the top)

 

This is very helpful. Thank you.

You should get to Thun (same train from Domodossola; arrives 11.58 (track 2)). That’s mainly because in Spiez the station is far away from the boat landing.

In Thun, you do just leave the station and cross the street and there’s the boat landing (about 1 to 2 minutes away). Boarding for the boat does start at 12.20. If you do hold a 1st class pass, there’s also a 1st class deck aboard the boat (upper deck). You may have a look at the boat here: Steamboat «Blümlisalp» | BLS-Schiff. 

The boat is covered with your global pass. You do just need to add it in your rail planner app like you do add every train (it’s in the timetable).

It does arrive at Interlaken West at 14.49 directly at the station. There’s a connecting train at 14.53 to Interlaken Ost from track 1 (possible to do; it’s a very small station and official connection time between boat and train is 2 minutes). If you don’t want to hurry: There’s a connection to Zweilütschinen via Interlaken Ost every 30 minutes. You should be in Interlaken Ost at 14.58.

Connecting train to Zweilütschinen is at 15.04; arrives 15.16. But as written before: Very frequent service.

Thank you. I have global pass 2nd class. I am understanding that I dont need reservation for all the legs from milan to interlaken. For the bus I will check in the milan train station probably the previous day if I have to pay any supplements. 
 

This has been very helpful. Appreciate all your help!! You guys are the best!