Several historical circumstances combined to end the Railroad Years and usher in the Automobile Years in Yellowstone.
The first automobile was allowed to travel from Gallatin to Yellowstone (current West Yellowstone) in 1913.
The Ranger Corps, (of two men) was established in 1915 (to kill coyotes, wolves, and cougars).
The Army, which had acted as the third arm of the railroads, left the park in 1916.
The National Park Service was established in 1916.
These circumstances and a host of others brought about great changes in Yellowstone Park, and in the perception of Yellowsotne Park by the people of the United States and the world. The automobile is a democratizing force. It is also a force for change that requires its own infrastructure. For a time there were competing modes of travel: by horse, by foot, by stagecoach, by railroad. Each required different accommodations, and each attracted different sorts of people. The transition was gradual because of the road system. Nevertheless the transition was complete.
As the automobile slowly gained ascendancy as the preferred mode of travel for visitors, the Park Service and the Ranger Corps adjusted their behaviors and rules, (more or less successfully,) to accommodate the individual and his independent itineraries. No longer could the railroad script the visitations. No longer could the Yellowsotne Park Company dictate travel time and meal time (and charge for both). No longer could the Army restrict travel to places near their posts, in order to control the visitors. The tail was no longer wagging the dog. Martial Law was a thing of the past (grudgingly so, according to several park superintendents, and many rangers who had been in the Army and enjoyed the mentality of policing rather than serving).
The transition was slow, and in some cases painful. Tough decisions were heaped upon an ill equipped infant Park service. Where should they put the roads (the old ones survived surprisingly intact)? Should gas stations be sited in the park? How big should parking lots be? Where should parking lots be placed? How were camping sites to be regulated? Should the Park Service regulate visitation and hotel construction? How should fees be structured? Who was entitled to the fees? Should free enterprise be regulated in the park? Should concessions and profit-generating enterprise be monopolized or diversified? Yellowstone National Park became, more so than before, the experiment - the test case for these questions, and many more.
Soon stage coaches were replaced by auto-busses, eventually the busses grew to diesel burning, exhaust spewing busses. The park had a special fleet of busses built. Enterprise outside the park began to offer specialized tours. More busses, vans, SUV's, and private automobiles entered the park. The Park service continued to struggle to accommodate the growing demand for individualized and scripted group access. Roads began to collapse, bridges widened and replaced, parking areas grew, pull-outs proliferated, and accommodations in the form of more souvenir shops, hotel rooms, lodges, and campsites bloomed around the park.
The railroads tenaciously held on to their rail service to Yellowstone Park. They only stopped service after W.W.II, and it became painfully apparent that they were losing enormous amounts of money to both the automobile and the nation's fascination with a new form of travel - the airplane. The Northern Pacific abandoned passenger service to Gardner, MT., in 1948, the Burlington Road abandoned its service to Cody, WY,. in 1956, the Union Pacific abandoned passenger service to West Yellowstone, Mt., in 1961, and the Milwaukee Road discontinued service to Gallatin Gateway MT., in 1961.
The railroad pullout also depleted funds for the Yellowstone Park Company, since the railroads could not see the need to subsidize a company that served auto travelers. The services in the park became a chaos of their own. The park and the Civil Aeronautics Administration briefly flirted with the idea of providing an air terminus in Yellowstone. When a study showed that 98.5% of the visitors arrived by automobile the idea died.
During the time between the wars, and during the period just after W.W.II, Montana and Wyoming concerned themselves with just how the park related to the states in terms of revenue, taxes, access, toll roads, and tourism dollars, (this state of affairs is not yet settled, though Wyoming still receives sales tax revenue from some concession establishments in the park).
By the end of the 60's and into the 70's the park was essentially a summer destination, visited by automobile. Although winter "keepers," rangers, and visitors had been in existence since about 1919, the discovery of Yellowsotne as a winter destination began to gather steam in the post war years and the invention of "snow-planes," and "snow-machines." During the 60's and into the mid 70's intrepid skiers, snow-planers, and snow-machiner's persisted in visiting the park. With the persistent demand from the public the park was again forced to contend with access during winter.
The administration of the park has progressed through several models as time and events have conspired to pressure the park personnel to respond to changing conditions. The great experiment that is Yellowstone [and our whole National Park System], has seen administrative models come and go. From the directive and edict administration of the very first superintendents, [some of whom were not even on site,] to martial law of the Army years, to congressional oversight during the war years, to the planning and study approach of the post war years, the park has struggled to keep up with the various demand placed upon it.