Thank you Larry, Tom, Michael, Jill and Connie for all your help and suggestions.
I was in Zermat in Jun 02 and I did layering like what you all suggested.. thermal inside (easily removed in washroom), a long sleeved Tee with a fleece jacket + gloves)and I was ok. Just that Jungfrau is higher and it's better to be safe than sorry!
You are all such great help.. for our trip, I was concerned about our warm clothing for our trip up to Jungfrau + at the same time, I didn't want to be slave to the clothing/luggage for the next 2 weeks just for that one trip up. I'm so glad this is sorted out now.. I was planning to leave 1 luggage (the Jungrau clothing) at the Interlaken rail station when we were done with it and have them transfer it to the Lucerne rail station while we continue on with the rest of our itinerary through Switzerland but was abit concerned in case of any mix-up/problem. What a relief!
We will get the light-weight windbreakers / jackets.. with that, we need not leave any luggage behind at the station. Thanks for info on the other stuff as well (hat, sunglasses, wool socks, sunblock etc.) Danke Schoen!!