Simplify the following text for a Grade 10 native English-speaking person.
Original: The Jubilee Line Extension , opened in three stages in 1999 , split from the existing line at Green Park , creating a one-station branch to Charing Cross , which is now closed ( although it is still sometimes opened for occasional use as a film set ) .
Simplified: The Jubilee Line Extension , as the eventual project became known , opened in three stages in 1999 . It split from the existing line at Green Park creating a one-station branch to Charing Cross , which is now closed ( although it is still maintained for reversing trains at times of disruption , and for occasional use as a film set ) .