The Auckland to Whangārei rail line upgraded from the nearly $165 million of taxpayers' money is scheduled to reopen in just under three weeks.
KiwiRail said once the line reopened on January 11, there may still be rail maintenance vehicles on the tracks at times so motorists were being urged to take care at level crossings.
The Government last year pledged $204.5m for the project— $164.5m to upgrade the Northland rail line and $40m to buy land.
The upgrade includes use of sleepers and ballast to renew or replace 54km of track, lowering the track in 13 tunnels so that standard shipping containers can fit through, replacing five bridges, and improving drainage.
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Northland contractors including Clements Quarries supplied ballast and Busck Prestressed Concrete provided sleepers used in the upgrade.
"With lowered tunnels and new bridges, the North Auckland line will be more reliable and have the capacity to take hi-cube containers. As well as reducing truck numbers, every tonne of freight carried by rail produces 70 per cent fewer greenhouse gas emissions than the equivalent freight carried by road," a KiwiRail spokesman said.
Currently 30,000 containers leave Northland each year by road and lowering tracks in the tunnels means many could be transported by rail.