By connecting the existing eastern U.S. rail networks to the west coast, the Transcontinental Railroad (known originally as the "Pacific Railroad") became the first continuous railroad line across the United States. It was constructed between 1863 and 1869.

The idea of a railroad that went from the east coast to the west didn’t start when building began. It is a story made up of a series of events and filled with the people and companies that made it happen—here are just a few of note:

• One of the early and most prominent people making the case for a transcontinental railroad was Asa Whitney. In 1849 he published his ideas on the idea of a railroad that began in Chicago and went to California. There were many others who also joined the chorus.
• In 1852 Theodore Judah was the chief engineer for the newly formed Sacramento Valley Railroad. He undertook a survey to find a manageable route through the high and rugged Sierra Nevada and in 1856 presented his plan to Congress.
• Congress passed the Pacific Railroad Act of 1862 on July 1, 1862, and the Central Pacific Railroad (CPRR) and the Union Pacific Railroad were authorized by Congress.
• One group of people that often get excluded from the discussion, but whom contributed much to the building of railroad were the thousands of Chinese workers. There was resistance to hiring them but eventually CPRR hired many to work on the railroad, but pay and treatment for them was not the same as it was for white workers. On June 24th, 1867 all Chinese railroad workers from Cisco to Truckee stopped work demanding higher wages and reduced work days. This protest lasted a week, but work continued and ultimately thousands of Chinese laborers died building the railroad. The Department of Labor inducted Chinese Railroad Workers into their Hall of Honor.

The rail line, also called the Great Transcontinental Railroad and later the "Overland Route," was predominantly built by the Central Pacific Railroad Company of California (CPRR) and Union Pacific (with some contribution by the Western Pacific Railroad Company) over public lands provided by extensive US land grants.

The railroad opened for through traffic on May 10, 1869, when CPRR President Leland Stanford ceremonially drove the gold "Last Spike" (later often referred to as the "Golden Spike") at Promontory Summit in Utah.

But the story of the railroads in the United States, and these two companies in particular, was really just getting started. The original Union Pacific, entangled in the Crédit Mobilier scandal and hit hard by the financial crisis of 1873, was eventually taken over by the new Union Pacific Railway in 1880 with its major stockholder being Jay Gould. It continued on, eventually becoming Union Pacific Railway. Central Pacific also went through changes including consolidation with the Western Pacific Railroad and the San Francisco Bay Railroad Co. under the name "Central Pacific Railroad Co." In 1885 it was leased to Southern Pacific and three years later the ICC listed it as non-operating. In 1899 it was reorganized as Central Pacific Railway and in 1959 it merged into Southern Pacific.