Complex sentence: No acts of sabotage occurred, but the 1941 rockslide at Johnsonburg was detected by such a watchman; the north side of Armstrong Cut was then trimmed back to prevent more rockslides.
Split into simpler sentences: No acts of sabotage occurred, but the 1941 rockslide at Johnsonburg was detected by a watchman. The north side of Armstrong Cut was then trimmed back to prevent more rockslides.