Will a Eurail pass cover all the trains for traveling from Zurich to Lauterbrunnen valley?
The Eurail pass will only cover to Interlaken, some discounts beyond that are available. More details here: http://www.ricksteves.com/rail/switzerland.cfm
Your Eurail pass will cover you to the Interlaken Ost train station which is the station you want for the transfer into the Lauterbrunnen Valley (there are two train stations in Interlaken). Beyond Interlaken Ost into and around the Lauterbrunnen Valley, I believe you get a 25% discount on the train tickets. I don't think that the Eurail pass is good on the bus. From Interlaken Ost, you will be on the Jungfraubahn train into Lauterbrunnen. Once in the valley, you will either be on the Jungfraubahn or Schilthornbahn train system. These are two private rail systems on the Swiss network. They don't seem to like the Eurail pass much as you only get a 25% discount.