Complex sentence: Originally was to be the Mandurah line, but an alternative route was taken from Perth along the Kwinana Freeway, now is a spur line built mainly to justify the tunnel work that was already completed.
Split into simpler sentences: Originally was to be the Mandurah line, but an alternative route was taken from Perth along the Kwinana Freeway. This route became a spur line to justify the tunnel work that was already completed.