Complex sentence: The roadbed westward from Livonia to the Atchafalaya River had been prepared by 1861, but the advent of the Civil War prevented the railroad from getting the necessary rails to complete the line.
Split into simpler sentences: The roadbed westward from Livonia to the Atchafalaya River had been prepared by 1861. The advent of the Civil War prevented the railroad from getting the necessary rails to complete the line.