Complex sentence: Although work had started in 1926, it was not until 1959 the Milson Deviation of the North Island Main Trunk was opened, meaning future trains will pass to the north of the city, instead of through the Square.
Split into simpler sentences: Although work had started in 1926, it was not until 1959 the Milson Deviation of the North Island Main Trunk was opened. This meant future trains would pass to the north of the city, instead of through the Square.