Complex sentence: The purchase included lands south of the Gila River and west of the Rio Grande; it was largely so that the U.S. could construct a transcontinental railroad along a deep southern route, and was constructed by the Southern Pacific Railroad in 1881/1883.
Split into simpler sentences: The purchase included lands south of the Gila River and west of the Rio Grande. The area was acquired largely so that the U.S. could construct a transcontinental railroad along a deep southern route, which was later completed by the Southern Pacific Railroad in 1881/1883.