The Swiss Travel Pass or the Eurail Pass can be used on those routes in Switzerland. 

The Swiss pass is also valid on buses, boats and public transport in cities. But it’s much more expensive.

For your journeys, I think a Eurail pass would be better, because it’s cheaper and covers both Italy and Switzerland. You’ll need reservation for the Eurocity train Switzerland to Milan, and for high speed & intercity trains in Italy.

I'd get Eurail too. Be aware that Interlaken Ost - Lauterbrunnen, Grindelwald and above is not covered but at least you get 25% discount. EC trains to Italy are quite small and popular so book in advance. Last-minute availability on Italian high-speed trains is common on the other hand : large trains and good frequency.

The Swiss Travel Pass is valid as far as Wengen, Mürren and Grindelwald and then you also get 25% discount for Jungfraujoch, etc.

Also possible would be to get the Swiss Half-fare (120 CHF for a month) + half-fare tickets/saver day passes. Then advance tickets in Italy. Probably good value if you intend to go to Jungfraujoch but then it requires more effort.

Dear thibcabe, thank you so much for your kind reply and advice. Hence I will buy Swiss pass and discount to jungfraujoch as well as eurail pass for Italy trains asap for our trip in late March to April.  

If you buy a Eurail global pass that covers all your travel days you don't need to buy a Swiss pass too.

Dear AnnaB,

Oh thank you for telling me I can also get Eurail global pass instead of swiss pass. I was thinking of combining both which is very costly. I will work out my math to see which one is cheaper. As I am in mid fifties so probably will go slow will not do alot of activities. So if getting Eurail global I can travel within Italy and Switzerland via trains with reserved seats more freely. 

Don't waste money on reservations in Switzerland. There is no need to make reservations there unless you absolutely want to travel in the panoramic coaches of the tourist scenic trains. The experienced travellers recommended you to use the regional trains that travel the same route as the tourist scenic trains but are less crowded and reservation free.