A transcontinental railroad in the United States is any continuous rail line connecting a location on the U.S. Pacific coast with one or more of the railroads of the nation's eastern trunk line rail systems operating between the Missouri or Mississippi Rivers and the U.S. Atlantic coast. The first concrete plan for a transcontinental railroad in the United States was presented to Congress by Asa Whitney in 1845.

A series of transcontinental railroads built over the last third of the 19th century created a nationwide transportation network that united the country by rail. The first of these, the 3,103 km (1,928 mi) "Pacific Railroad", was built by the Central Pacific Railroad and Union Pacific Railroad to link the San Francisco Bay at Alameda, California, with the nation's existing eastern railroad network at Council Bluffs, Iowa/Omaha, Nebraska, thereby creating the world's first transcontinental railroad when it opened in 1869. Its construction was made possible by the US government under Pacific Railroad Acts of 1862, 1864, and 1867. Its original course was very close to current Interstate 80.
Extract the names of the companies that built the Pacific Railroad
The "Pacific Railroad" was built by the Central Pacific Railroad and Union Pacific Railroad.