Hi James not sure if this will help, but I stayed in Lauterbrunnen for 4 nights just over a year ago. I bought a swiss pass before I left home. While I normally do not believe in rail passes value, I thought the swiss pass was a great deal. I was suprised to recently read in Rick's Best of Europe that the rail pass can not be used any higher then Interlaken.
When I was there this was not the case at all! I traveled with my rail pass from Interlaken - Lauterbrunnen. Then the TI in Lauterbrunnen gave me a map with all the places I could use my pass at no extra cost. I was able to use my swiss eurail pass to go from Lauterbrunnen to Murren, Grindelwald, Gimmelwald, Wengen, and even up to Birg (that was a mistake and I was sent back down)! The pass was also good for the buses and boats. Besides Birg, Shilthorn, and Jungfrau I got around the area great with my pass. When I needed info the attendents at the station were great, I just showed them my pass and they told me how much extra Shilthorn and Jungfrau would be, with the pass you save 50% of the tickets.
Not sure if this has changed since I was there as I said it has been just under 2 years. I think you should be fine with just a Swiss pass.