Complex sentence: The railroad then built southwest from Benson, Arizona, to Nogales on the Mexican border where it connected with the Sonora Railway, which the AT&SF had built north from the Mexican port of Guaymas.
Split into simpler sentences: The railroad then built southwest from Benson, Arizona, to Nogales on the Mexican border. There it connected with the Sonora Railway, which the AT&SF had constructed north from the Mexican port of Guaymas.