Complex sentence: The line was scheduled to open in two stages: the first stage from Epping to Hills Centre Station was scheduled for completion by 2015 (originally 2017), and the second stage from Hills Centre to Rouse Hill Station was scheduled for completion by 2017.
Split into simpler sentences: The line was set to open in two stages, with the first stage from Epping to Hills Centre Station completed by 2015 (originally set for 2017). The second stage from Hills Centre to Rouse Hill Station was due by 2017.