Source: United States National Archive.
The completion of the transcontinental railway in 1869 marked a turning point in the territorial development of the United States. The last spike was driven in to join the rails of the Union Pacific and the Central Railroad Companies on Monday, May 10, 1869, at 12:45 PM at Promontory, Utah. The East Coast was finally linked to the West Coast by land transportation, allowing for the continent to begin to function as a whole market and supporting the specialization of its regions.