Why was the United States willing to pay more per mile for the Gadsden Purchase than for the Mexican Cession?

The Gadsden Purchase included a strip of land in present-day southern Arizona and New Mexico that provided a suitable route for a southern transcontinental railroad. Because this land was especially valuable for the planned railroad, the United States paid more per mile for it than for the much larger Mexican Cession. The correct choice is the land for building a railroad.

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