Trains will work just fine for this itinerary, no need to fly. You just need to book your Eurostar seats in time, especially the one from London.

You can save some money on reservations:

Amsterdam-Paris: use regular intercity to Lille, then TGV.

Paris-Geneva: TGV to Bellegarde, then TER.

Geneva-Interlaken: no reservations needed in Switzerland, that's a waste of money.

Reservation fees are indeed quite high on the first few trains: €35 for London to Amsterdam, €27 to Paris, €29 to Geneva. But @rvdborgt has given you some suggested cheaper itineraries.

No reservations needed in Switzerland. 

Thereafter, it’s not too bad: Switzerland to Milan €13, and all high speed trains in Italy €13 each.
 

I would consider instead buying a ‘4 day in one month’ pass, then just buy cash tickets for the two other journeys. You can check prices for all these journeys on www.thetrainline.com (Not necessarily the best place to buy tickets, but a good place to quickly check prices)

Switzerland to Milan can be done for €10 by booking from Domodossola. Reservations are only required from there.

Thank you everyone.

so I am thinking to fly from London to Amsterdam. Get 7 days in  a month pass for trip to

• Amsterdam to Paris
• Paris to Geneva
• Interlaken to Venice
• Venice to Rome

Not sure if pass would be cheaper around Interlaken area ( like Interlaken to Zermatt). If it is not, I would rather buy 4 days pass and buy local train tickets.

Any suggestions?

​​​​​There are still pass holder seats available for London-Amsterdam on 27 August. Travel time is just over 4.5 hours. I doubt flying would be faster if you take into account getting to and from the airports.

Swiss trains are not cheap (see sbb.ch). Interlaken-Zermatt is CHF 86 for a single. Geneva-Interlaken is CHF 75. The pass is excellent value for Switzerland.

@rvdborgt, you are amazing.

So buying 10 days in  2 months pass worth it? Comes to be around 400 USD plus reservation cost.

You want to use some travel days while you’re in Interlaken, I guess? How many days?

Did you look at prices on the Trainline? There are plenty of trains available between Paris-Geneva on 30/8, costing between €29 and €64, depending on departure time. For most of these trains, it would be better to buy. a ticket, rather than spend a pass day + pay €29 reservation fee.

Similar for Amsterdam - Paris. Depending on what time you want to travel, it might be better to buy a ticket.

If you're prepared to fix the schedule (since advance fares can't be refunded or changed), then you should check all fares and see for which days a pass makes sense (if any).