Which train pass the best

Maybe you could describe 'the Grindelwald trail" for us so we understand your plan. Is this a multi-day hut-to-hut hike, or just general hiking in the Grindelwald area.

A multi day unguided hike staying in inns and small hotels. Includes Interlaken, Grindelwald, Wengen and Murren. Onto Zermatt.

Will you be needing to use lifts during time?

Yes, trams, gondolas and funiculars

If you get the Half Fare Card, you get a discount of 108 CHF off the 8-day Berner Oberland Pass, giving you access to all the lifts in the region except the Jungfraujochbahn above Eigergletcher and the Schilthornbahn above Murren, but you get a discount on these. So in effect, your HFC costs you 12 CHF. That will give you half off the train from Geneva Airport, the train to Zermatt, the lifts in Zermatt, and the train back to Geneva Airport.
https://www.berneseoberlandpass.ch/prices-tickets/

@Sam has given you excellent advice on using the HFC and BO Pass. I had never heard of the "Grindelwald Trail" before, despite living here for many years, so I looked it up.

It appears that "Macs Adventures" offers a package where they arrange your overnight stays along a loop of hiking trails they’ve selected.

The only required transportation in their itinerary is from Interlaken Ost to Schynige Platte, where your hike begins, and returning to Interlaken from Mürren at the end.

You can find the cost for these routes on the SBB website or app:

• Interlaken Ost to Schynige Platte: CHF 17.90 with HFC, CHF 35.80 full fare
• Mürren to Interlaken Ost: CHF 9.80 with HFC, CHF 19.60 full fare

You’ll have a free day in both Wengen and Mürren, allowing you to choose your own local hikes.

Whether you hike the entire route or use the available lifts will determine if the BO Pass is necessary; it's only useful if you plan to take the available lifts (adding up to more than 35.00/day for an 8 day pass with the HFC).

Thanks for this great idea. Should the passes be bought online prior to trip or at a TI upon arrival.

I think from the results of SwissNomad's comments, it sounds to me like a Berner Oberland Pass would be overkill. If you are doing the Mac's Adventures trip, a Half Fare Card at 120 CHF per person is all you really need, and you can buy that when you arrive at Geneva Airport.