Auckland Council says that Auckland is growing at a rate of one new resident every 20 minutes and urgently needs a transport system that moves people faster and more easily.
Rail trips have increased from 2.2 million trips a year to more than 16 million, but further growth is limited because Britomart can't take more than 20 trains an hour.
In addition, the city is over capacity for buses coming into the city along Symonds Street.
The City Rail Link (CRL) will unlock the city's rail capacity, doubling the number of trips an hour during peak times and providing a train every 10 minutes for most Auckland stations.
It will allow 30,000 people to travel every hour during peak time, up from 15,000 currently. In comparison, a single motorway lane can carry only 2400 people an hour.