Complex sentence: The purchase included lands south of the Gila River and west of the Rio Grande which the U.S. acquired so that it could construct a transcontinental railroad along a deep southern route, which the Southern Pacific Railroad later completed 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.