The Pacific Railroad Surveys (1853–1855) were a series of explorations of the American West designed to find and document possible routes for a transcontinental railroad across North America. The expeditions included surveyors, scientists, and artists and resulted in an immense body of data covering at least 400,000 square miles (1,000,000 km²) on the American West. "These volumes... constitute probably the most important single contemporary source of knowledge on Western geography and history and their value is greatly enhanced by the inclusion of many beautiful plates in color of scenery, native inhabitants, fauna and flora of the Western country." Published by the United States War Department from 1855 to 1860, the surveys contained significant material on natural history, including many illustrations of reptiles, amphibians, birds, and mammals. In addition to describing the route, these surveys also reported on the geology, zoology, botany, paleontology, climatology of the land as well as provided ethnographic descriptions of the Native peoples encountered during the surveys. Importantly, a map of routes for a railroad to the Pacific, was compiled to accompany the report.

Starting in the late 18th and early 19th centuries, many Americans began a westward migration that would come to greatly influence the development of American history. However, water travel remained the most common and most efficient form of transit available. Soon, the development of the steam engine became an invaluable contribution to this westward expansion. As railroads gained popularity in the eastern United States during the 1830s, Americans felt an increased incentive to expand this new technology to the western frontier.

Beginning in the 1840s, several government sponsored expeditions hoped to find potential railroad routes across the west. However, no consensus route emerged due to the selfish economic motives of rival companies. In addition, cities and states competed for the route and terminus so no consensus was reached. Brigham Young, President of LDS Church, wrote, "We never went through the canyons or worked our way over the dividing ridges without asking where the rails could be laid." On March 3, 1853, Congress appropriated $150,000 and authorized Secretary of War Jefferson Davis “to Ascertain the Most Practical and Economical Route for a Railroad From the Mississippi River to the Pacific Ocean.” Davis ordered Brevet Captain George B. McClellan and the Corps of Topographical Engineers (TOPOGS), a division in the United States Army established to “discover, open up, and make accessible the American West,” to fulfill this obligation.

The most important concern for the United States Congress involved the location of where to build the railroad. With government involvement, lobbyists attempted to influence the selected locations because of the important social, political, and economic consequences. In addition, a transcontinental railroad would become a very costly endeavor. In fact, “Even the least expensive proposed routes would equal the federal budget for one year.” Despite these obstacles, a developing urgency clearly indicated the need for a transcontinental railroad. On August 16, 1856, Mr. Denver of the House Select Committee on the Pacific Railroad and Telegraph reported that: "the necessity that exists for constructing lines of railroad and telegraphic communication between the Atlantic and Pacific coasts of this continent is no longer a question for argument; it is conceded by every one."

The path of the first transcontinental railroad route was one of many proxy fights over the future of slavery in the United States exacerbated by capitulation and Mexican Cession resulting from the Treaty of Cahuenga and the Treaty of Guadalupe Hidalgo. The Compromise of 1850 which admitted California as a slave-free state, defined the geographical boundary of Texas as a slave state, banned the slave trade (but not slavery itself) in Washington DC, enhanced the Fugitive Slave Act, and most relevantly established Utah and New Mexico territories under popular sovereignty — meaning whether any future state from these territories would be free or slave would be decided by the constituency of migration at the time of the petition for statehood. The route of the first transcontinental railroad would determine whether slaves could be legally and efficiently trafficked into these geographically isolated territories. So the route was not just ancillary to free soil policy, but could ultimately affect the balance of power between the north and south in Congress when new states were inevitably admitted into the Union out of these regional territories. Just three years after this compromise, Jefferson Davis strongly influenced the Gadsden Purchase to facilitate his preferred southern route which was not viable over the Colorado Plateau. A year after this purchase, Bleeding Kansas began on account of identical thematic tensions. The Kansas–Nebraska Act was intended to open up new lands to develop and facilitate the construction of the transcontinental railroad, however it effectively repealed the Missouri Compromise of 1820. The map of the Pacific Railroad Surveys would be published in 1855.

The congressional inaction in the survey's aftermath is a reflection of the severity of this proxy fight. Despite the United States having common carrier railroad infrastructure since the 1830's, Congress was politically unable to enact any decision on an initial transcontinental railroad route until the south seceded from the Union in December 1860 in response to the November 6th presidential election. Contrasting national division along party lines on the issue, both major California political parties included a Pacific Railroad as part of their platform. That week and month, Theodore Judah, who himself had been engineering railroads since the 1840's, and California railroads in particular since the 1850's, finally obtained the political opportunity for a central route with Abraham Lincoln party associate and elector Leland Stanford. Slavery operative Jefferson Davis became President of the Confederate States of America. The Pony Express and then the first transcontinental telegraph were also initiated at this time out the urgency of the U.S. Civil War. Congress passed the first of the Pacific Railroad Acts and the major Homestead Act in 1862. The Central Pacific Railroad then broke ground on January 8, 1863. Though the last spike would not be driven into the transcontinental railroad until 1869, the second transatlantic telegraph cable was completed the year the Civil War ended.

Five surveys were conducted.

Although the Pacific Railroad Surveys (1853–1855) provided valuable information regarding the possible routes for the transcontinental railroad, they were not compelling enough to lead directly to construction. Three important trends also influenced Congress’ final decision. First, the California Gold Rush and the discovery of silver in Nevada led to a dramatic increase in population in the west. Second, the secession of the South from the Union during the beginnings of the American Civil War discounted southern politicians from interfering with a plan to build a northern or central route. Third, a growing population of railroad specialists allowed Congress several options to consider the most efficient and cost effective route to build a transcontinental railroad.