Complex sentence: CMR operates over an active portion of the former Chicago, Rock Island and Pacific Railway's St. Louis to Kansas City, MO, main line that was constructed in 1870, which now terminates in Union, MO, approximately 36 miles west of Chesterfield.
Split into simpler sentences: CMR operates over an active portion of the former Chicago, Rock Island and Pacific Railway's St. Louis to Kansas City, MO, main line that was constructed in 1870. Active service now terminates in Union, MO, approximately 36 miles west of Chesterfield.