<P> The report failed however to include detailed topographic maps of potential routes needed to estimate the feasibility, cost and select the best route . 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 and New Mexico . This in part motivated the United States to complete the Gadsden Purchase . </P> <P> 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: </P> <P> 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 . </P> <P> The U.S. Congress was strongly divided on where the eastern terminus of the railroad should be--in a southern or northern city . Three routes were considered: </P>

Why did congress support the building of the transcontinental railroad