By the early 1900s, the Union Pacific Railroad's subsidiary rail line - the Oregon Short Line - reached Ashton, Idaho. In 1905, E.H. Harriman, President of the Union Pacific Railroad, traveled through Yellowstone National Park accompanied by Frank Haynes to meet with officials from the Northern Pacific Railway. They likely discussed the increasing need for more hotel facilities to serve the growing numbers of Yellowstone visitors. Following his visit to the park, Harriman immediately ordered construction of railroad tracks from Ashton, Idaho to the park's western border in Montana.By 1906, the line stretched to Big Springs, Idaho. The following year, tracks were laid over the Continental Divide into Montana by way of Reas Pass. The route followed the South Fork of the Madison River to the flatter lands of the basin, then swung directly east toward the park. On November 12, 1907, the last rail was laid next to the park boundary.
On June 11, 1908, the first passenger train steamed into West Yellowstone, then known as "Boundary." That first year, a wooden depot stood as the lone railroad facility at the park terminus. Construction on the stone depot (now home to the Yellowstone Historic Center Museum) started later that year and was completed in 1909. By 1929, the Union Pacific's facilities included the massive stone dining lodge, dormitories, a baggage building and the water tower.
Passenger trains only serviced West Yellowstone from mid-June through mid-September. Winter snows buried the section of tracks from Ashton to West Yellowstone to such depths that it was impossible to keep the route open. Each year, starting in March, the "Spring Campaign" was underway to clear the snow from the tracks. Wedge and rotary plows muscled through snow depths of 15 feet or more for several weeks. The eager townsfolk of West Yellowstone welcomed the sight of the first train of spring. They held a celebration with dancing and revelry in the local school gymnasium in honor of this symbolic end of another winter.
Initially, train passengers continued their journeys into the park aboard stagecoaches. In 1916, the Yellowstone Park Transportation Company gained exclusive rights to the park's public transportation. The following year, they premiered their fleet of White Motor Company touring buses, eventually known as the "yellow buses." The peak years for train travel to West Yellowstone were the mid-1920s through the early 1930s, with three to four hundred passengers arriving daily.