Editing First transcontinental railroad (section)

===Preliminary exploration===
[[File:TranscontinentalPoster.jpg|right|thumb|upright|The official poster announcing the Pacific Railroad's grand opening]]

Congress agreed to support the idea. Under the direction of the [[United States Department of War|Department of War]], the [[Pacific Railroad Surveys]] were conducted from 1853 through 1855. These included an extensive series of expeditions of the American West seeking possible routes. A report on the explorations described alternative routes and included an immense amount of information about the [[American West]], covering at least {{convert|400000|sqmi|km2|abbr=on}}. It included the region's natural history and illustrations of reptiles, amphibians, birds, and mammals.<ref>[http://www.cprr.org/Museum/Pacific_RR_Surveys/ "Reports of Explorations and Surveys, to ascertain the most practicable and economical route for a railroad from the Mississippi River to the Pacific Ocean, made under the direction of the Secretary of War, in 1853–4."] 12 Volumes. Washington, DC: US Government Printing Office, 1855–61</ref>

The report did not include detailed [[topographic map]]s of potential routes needed to estimate the feasibility, cost and select the best route. However, the survey was detailed enough to determine that the best southern route lay south of the [[Gila River]] boundary with Mexico in mostly vacant desert, through the future territories of [[Arizona Territory|Arizona]] and [[New Mexico Territory|New Mexico]]. This in part motivated the United States to complete the [[Gadsden Purchase]].<ref>Woodward, C. Vann "Reunion and Reaction: The Compromise of 1877 and the End of Reconstruction". Oxford: Oxford University Press (1991) p. 92</ref>

In 1856, the Select Committee on the Pacific Railroad and Telegraph of the US House of Representatives published a report recommending support for a proposed Pacific railroad bill:

{{Blockquote|The necessity that now 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. In order to maintain our present position on the Pacific, we must have some more speedy and direct means of intercourse than is at present afforded by the route through the possessions of a foreign power.<ref>[http://cprr.org/Museum/HR_Report_358_1856.html "Report of the Select Committee on the Pacific Railroad and Telegraph"] US House of Representatives, 34th Congress, 1st Session, No. 358. August 16, 1856.</ref>}}