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With the rise of newer and heavier locomotives and consequent increase in tonnage of goods transportation, the gradient to reach the Waitakere Tunnel was proving to be a hindrance as early as the 1920s. Trains leaving Swanson would drop down to the old timber-piled bridge and then had to climb steeply to reach the tunnel. Work on the grade easement scheme began in the mid-1930s as a relief labour scheme but stopped due to the war. Another 40 years passed before the climb from the Swanson Stream bridge to the Waitakere Tunnel was eliminated from 3 October 1981.
Apart from removing the level crossing over the Scenic Drive and the old bridge over Swanson Stream, the deviation allowed heavier northbound trains to be hauled between Mount Eden and Helensville. Easier curves on the new line enabled trains to run somewhat faster than before.