We will be staying in Interlaken but want to travel to Lauterbrunnen/Murren. We have a Eurail Pass. Will we have to lose a day of the Eurail Pass when purchasing a dicounted ticket to go to Lauterbrunnen/Murren??
Eurail passes are not valid on BOB. You will have to anypay pay from Interlaken to Lauterbrunnen and Murren irrespective of the Eurial pass.
So no worries!!!
I presume that you are using a Eurail Flexi pass, as you mention using a day of your pass.
No, you do not need to use a day of your pass to get the discount - the pass just needs to be valid on that day. So, for example, if you have a 10 travel days in two months pass, you can get the discount so long as it is within the two months validation period of the pass. Now, that being said, I was never once asked to show my pass to get the discount at the ticket booths.
You can get the discount both on the train from Interlaken to Lauterbrunnen, and again on the gondola up to Murren.
Just as an aside, if you had the Swiss rail pass, it would completely cover the transit from Interlaken up to Murren, but if you are using something like a global pass then I can understand why you would not have gone with a separate swiss pass.
Have a great trip!