What was the purpose of the gasden purchase?

The Gadsden Purchase included lands south of the Gila River & west of the Rio Grande. It's purpose was to aquire land for the construction of a trancontinental railroad along a deep southern route. It was the last major territorial transaction in the lower 48 states. It is named for James Gadsden who was the ambassador to Mexico at the time of the purchase in 1853. The US paid $10 million for this addition which is now parts of the present day states of Arizona & New Mexico.