Review 3 day’s Itinerary

Do we need to acclimatise for Jungfrau ?

I honestly don't know how you would do that. If you do go, you will only be there for about an hour, from the sound of everything else you have planned for that day.

Also do we need to book seats on EU rail pass

Only Swiss IC trains even have seat reservations available. Very few Swiss bother with these as the frequency of the trains.
You can get seat reservations for the Jungfraujoch, but how do you know if it will even be worth going on your chosen day? Note that your Eurail pass only gives you a 25% discount from Interlaken to the Jungfraujoch and back. It can get socked in frequently. You can also get seat reservations for the GP from https://www.gpx.swiss/en/pages/book-gpx-seat-only

thanks for the response sam

we have booked Jungfraujoch for 4 hrs, i was not sure how much time we will spend on the top. Although natively i am a person from the mountains however off late i have been living at sea level hence wanted to check.

You are going from Biel at 1400 ft, to the Jungfraujoch at 11400 ft, a 10.000 ft elevation rise in a matter of hours. How do you acclimatize for that? If you feel sick, head back down.

Have you calculated how much time you're spending doing this plan?

Day 2 - 3.5 hrs each way to Jungfraujoch from Biel = 7 hours
Day 3 - 5.5 hrs to Montreux via GPE and almost 2 hours back to Biel = 7.5 hours

The Golden Pass Express is not the most scenic train in Switzerland and you're spending way too much time doing it. Trying to see this much of the country in 2.5 days just isn't realistic. It sounds like you've already booked Jungfraujoch, so you can certainly keep that plan but I would change Day 3 to something more realistic like a day on Lake Thun, Lake Brienz or Lake Luzern.

Hi there, I recently moved to the Seeland Region, and it is a treck to go to the Jungfrau Region from here - but certainly doable. As you say, you will leave early in the morning. You can grab something at the train station to eat and drink along the way, sleep a bit if you are tired, enjoy the view as you enter the Berner Oberland at Thun. If it is something you want to do, don’t let the travel time stop you.

I will say that personally I don’t find Biel that interesting. You will be very close to Neuchâtel (19 min.), I would spend some of Day 1 there. Also very reachable by train is Murten (52 min.). Both have beautiful lakeside walks as well as very interesting old town areas to explore.

If you are interested in watches you can go up to La Chaux-de-Fonds (41 min.) https://www.myswitzerland.com/en-ch/experiences/on-the-trail-of-watches/

The Seeland (Lake Region, or Three-lakes Region) is extremely popular for cycling. You could rent an e-bike and explore that way. Cycling routes can be found here:

https://schweizmobil.ch/en/cycling-in-switzerland

Montruex can be reached in 1.5 hours, you can easily spend a day exploring that area. There is also Gruyères (2 h) where you can walk through the village, explore the castle, see the cheese making then head up the local mountain https://www.moleson.ch/en/ Lots of fun activities there. Stop at Cailler chocolate museum in Broc on your way back to Biel.

Edited to add that this is an area where having a car makes a lot of sense. You can drive Biel - Interlaken in just over 1 hour, but it seems you already have your train pass, so perhaps this is a moot point.