Complex sentence: The extension of the Flushing Line, whose trackwork was completed in late 2013, saw third rails being added, and the tracks inspected or replaced before they are opened to revenue service.
Split into simpler sentences: The extension of the Flushing Line, whose trackwork was completed in late 2013, has work being done to improve this stretch. Third rails will be added, and the tracks inspected or replaced before they are opened to revenue service.