What was the outcome of the Gadsden Purchase?

The Gadsden Purchase provided a route for the Southern Pacific Railroad.
After the Mexican War, the southern border of the United States did not assume its current configuration. What we know of today as roughly the southern third of the state of Arizona (south of the Gila River) and the little panhandle of the state of New Mexico (due west of El Paso, TX) was still Mexican territory.
As railroad interests were planning for the westward expansion of rail transportation to California, there was pressure for a more southern route for a rail line that would not have to deal as much with cold, snowy mountain peaks. Business interests felt that the best route across the southern US had to dip into this section of Mexico and there was pressure on the government to purchase it so that there were no international issues involved in its construction. In a purchase agreement finalized in 1854, President Franklin Pierce formalized the purchase of this tract of land.
The secession of the south and the coming of the Civil War delayed the beginning of construction of this rail line, but it was finally completed in January of 1885, marking the opening of the second transcontinental railroad line.