Complex sentence: The ferry was trying to maintain position to retrieve a body when her stern gates were overpowered by waves; she sank within 30 minutes taking several rail cars and 4 crew members, including the Captain, to the bottom of the Cabot Strait.
Split into simpler sentences: The ferry was trying to maintain position to retrieve a body when its stern gates were overpowered by waves. It sank within 30 minutes taking several rail cars and 4 crew members, including the Captain, to the bottom of the Cabot Strait.