Southwest Airlines on Friday began flying its first jetliner built with a secondary flight deck barrier designed to prevent intrusions in a design change that's been in the works since the Sept. 11, 2001, terror attacks. The airline took delivery of a Boeing 737 Max 8 in recent days that flew from Phoenix to Denver and the flight serves as a milestone for a safety feature being deployed across new commercial airliners entering U.S. service in the coming years.Secondary barriers have been sought since Sept. ...