A detailed study is to begin into the route of a possible rail tunnel beneath downtown Auckland.
KiwiRail and the Auckland Regional Transport Authority (ARTA) will jointly consider an underground route to link Britomart with Mt Eden railway station.
The estimated cost is more than $1 billion.
The twin-track tunnel would allow trains to travel in a loop out of the central city, rather than having to backtrack through a single-track tunnel that feeds Britomart.
Ontrack says a tunnel is an unobtrusive means of unlocking Britomart as a gateway to the city.
Consultants will spend the next 12 to 15 months assessing various options.
Two underground stops will be considered, including a station 30 metres beneath Karangahape Road.
Chief executive Fergus Gammie says the tunnel would have the biggest transport impact on the central city since the opening of the Harbour Bridge 50 years ago. He says it will change the entire scale of rail usage in Auckland.
