I will be visiting Swiss late May / Early June with my wife and 5 yrs old Kid , would like to get recommendations on the best express Train to travel and View the Swiss Mountains.
Express train from Gindelwald? There is a train station, but you will already be right in the middle of the Swiss Alps. You don't need a train to see the beautiful mountains. Just step outside your hotel door and there they will be.
This is what you will see from Grindelwald (photo): https://image.jimcdn.com/app/cms/image/transf/none/path/s888dd5d66abee7a9/image/ice6975b549c557cc/version/1433338843/image.jpg
Look at a rail map of Switzerland: https://www.sbb.ch/content/dam/sbb/de/pdf/freizeit-ferien/ferien-kurztrips-schweiz/internationale-gaeste/sts-geltungsbereich.pdf
Grindelwald is at the end of a dead end valley. You have mountains all around, and mountain railways and cable cars to take you up them. You take a local train from Interlaken to reach Grindelwald, no expresses.
You could always take the Wengneralpbahn up over the top at Kleine Scheigegg and down to Lauterbrunnen. Return via Zweiluetschenen on the Berneroberlandbahn. They are hardly "express" but on a nice day, quite scenic.