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The main south rail line is expected to reopen today after close inspection of the track revealed considerable damage following a mass derailment on Wednesday.
The southbound freight train was hauling 20 wagons when it derailed on straight tracks between Palmerston and Bushey about 5am, with nine wagons derailing and forced on their sides.
A KiwiRail spokeswoman said repairing the track was originally expected to take 24 hours but a closer inspection ''found considerably more track damage than first estimated''.
About 20 services had been affected, with extra capacity in the form of services and possibly extra wagons expected to be added today, she said.
The line was expected to be operational from noon.
It was ''too early to speculate as to the cause'', but preliminary investigations suggested failings with a New Zealand-made wagon, and an investigation was under way.