The Southern Transcontinental Railroad
The South wanted a better way to trade and wanted a stronger connection to the western territories as well as to follow manifest destiny and moves westwards.. Because of this, they thought of have a Transcontinental Railroad connecting the south with the west coast. There was one problem though, the mountains. The terrain made it hard to create a pathway that would not veer north. The southerners were afraid that if the railroad would come north, it would also give them a strong trading presence. With different interests, the south did not want to do anything that would give the north a upper hand.
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The only other viable route for the Southern Transcontinental Railroad would cut through part of Mexico. The extra land that the Gadsden Purchase would eventually provide would be needed to complete the railroad path and strengthen the South's trading presence.
The Gadsden Purchase would let there be a viable route for the Southern Transcontinental Railroad which would help the South. It would take around 20+ years before the Railroad would be fully built. Read More Here