From a National Park informational plaque:

In the early days of Glacier, visitors experienced the park from the saddle of a horse. When gravel roads were built, bumpy horseback rides were replaced by equally bumpy automobile rides. Beginning in 1914, White Motor Company touring buses began taking passengers through the park.

The buses that ornament the roadways of Glacier today are part of the largest, longest-running fleet of vehicles in the National Park Service. However, in the late 1990s, after six decades of sightseeing tours, the fleet showed serious wear and had become a safety concern. In 1999, the red buses were taken out of service. Through the generous support of the Ford Motor Company and other partners, the busses were refurbished, each getting a new V8 bi-fuel engine and new chassis atop its original wheel base. They returned three years later--safer, cleaner, environmentally friendly, and more comfortable, still retaining their classic look and feel, and continuing a long standing park tradition.