Hello jamesmorrone!
The Brienzer Rothorn is covered by the Regional-Pass Bernese Oberland, not by the Jungfrau Railways Pass. You can best download the validity maps from the pages I linked to in order to see where the passes are valid.
Planning your trip is much easier with a free MySwissAlps account. Learn all about Switzerland and get in touch with us quickly if you have questions.
Let Swiss travel consultants create a customized itinerary. They will book your entire trip.