Underwood used rhyolite from nearby quarries and local timber for the new Dining Lodge. The inside consists of a large, open room lined with rhyolite stones and a roof supported by timber beams. At the far end of the hall is a massive, arrowhead-shaped fireplace, also made of rhyolite. Windows that stretch nearly the height of the building are located all along the south of the building, part of the north wall, and on either side of the fireplace at the west end, allowing for natural sunlight and views of the outdoors. The room was built to seat up to 350 diners at once. Fearing this still would not be enough, the former rest pavilion was rotated 90 degrees, enclosed, and attached to the east end of the new structure to serve as a waiting area.
In 1960, the Union Pacific ended passenger service into West Yellowstone due to years of decline in travel by rail. At this time, the Dining Lodge was no longer needed and closed. In 1966, when West Yellowstone was officially incorporated the Union Pacific deeded their abandoned structures to the Town of West Yellowstone. Since then, the building has been used for a variety of purposes, including housing the Public Library from 1981 until 2007. Today, it is used as an event center, serving as a venue for weddings, gatherings, celebrations, and more. The main room, known as the Mammoth Room, and the former waiting area, known as the Firehole Room, have been mostly untouched since 1925 and continue to leave a lasting impression on all who visit.