The main barrier to a rail link was the crossing of the Manukau Harbour between Onehunga and the suburb Mangere Bridge. Transit New Zealand announced in 2007 that a new motorway bridge duplicating, and alongside, the existing 1983 Mangere Bridge would be designed to accommodate a rail link. In September 2007, Auckland Regional Transport Authority announced that it was willing to pay Transit $2.5 million to "future-proof" the duplication of the bridge in order "to accommodate a passenger rail connection". The duplication of the bridge was completed in 2010. The new bridge can technically accommodate a rail link, however this is a curved, single-track alignment with a design speed of 20 km/h. The successor to Transit New Zealand, NZ Transport Agency, provided for a rail corridor near the motorway as far as Walmsley Road. The proposed line was the most popular with the public of the three Auckland rail proposals of Mayor Len Brown, although the number of trains would be limited without the City Rail Link to Britomart Transport Centre. Mayor Brown had promised a rail link to the Airport during his 2010 election campaign as part of a package of measures to double public transport patronage within 15 years. The official name for the airport link in the Long-term Plan 2012–2022 is the South Western Airport Multi Modal Corridor Project. The Airport rail link planning study commenced in 2010. The Auckland Airport company asked for a decision by mid-2016 whether a rail or bus station would be needed at the airport, as it was planning a new domestic terminal, with construction to start in 2021. Escalating costs for the heavy rail line to the Airport prompted some to raise the possibility of the route being served by light rail between Onehunga and the airport as an alternative to heavy rail. This option would make use of existing infrastructure where possible to reduce expenses and would involve an interchange with the existing heavy rail services at Onehunga.
Earlier heavy rail proposal
The line was initially conceived as an extension of the Onehunga Branch line over the Mangere Bridge and via the suburbs of Mangere Bridge and Mangere. However the 2016 study identified this as an expensive option:
the high cost of grade-separating and closing level crossings on the branch line versus the less expensive but much less safe option of no grade-separation with barriers and pedestrian gates
the extent of viaducts and tunnelling required for an underground terminus under the airport terminal, with a decision needing to be made in time for the airport to begin construction of a new domestic terminal
These made an extension of Auckland's heavy rail network an expensive option. This heavy rail option would add three new stations and require airport trains to run via the already-congested tracks and junctions of the inner Southern Line. The heavy rail option would result in a journey time of 39 minutes from the airport to Britomart Transport Centre in Auckland CBD, at an estimated cost of NZ$2.2 billion. The choice of mode and alignment for the Airport Line remains contentious. In the 2017 general election campaign, promises of light rail to the airport were made by the Labour Party, which established the Sixth Labour Government with a coalition agreement. One coalition partner, the Green Party, also favoured light rail although the other partner, New Zealand First, favoured the earlier heavy rail scheme. The previous government said that light rail would be eventually required, but would not happen for thirty years.
Route and choice of mode
The preferred option for the proposed Auckland Airport Line, supported by a 2016 business case study, is an entirely new light rail transit line running between the airport and the Auckland central business district. From the airport end it would run on a direct alignment through Mangere and Onehunga to Mount Roskill, where it would connect to the planned Dominion Road light rail line, which would continue on to Queen Street in central Auckland before reaching a final terminus in the Wynyard Quarteron the waterfront. With this alignment approximately two-thirds of the line would run on a grade-separated light railway, while one-third of the route would run via dedicated lanes at street level along Dominion Road and Queen Street. Dominion Road would be modified to ensure fast travel times and high reliability; the proposal includes converting the current bus lanes into kerb-separated, dedicated light rail lanes running in the middle of the street, signal priority for light rail at major intersections, and introducing right turn bans for traffic at smaller intersections and side streets. Queen Street would be converted into a pedestrian and light rail-only transit mall. The combined light rail option would add fifteen new stations across the airport, southwest, isthmus and waterfront areas of Auckland, and have a journey time of 44 minutes from the airport to Britomart. The combined cost of the Dominion Road light rail line and the airport extension is estimated at NZ$2.0 billion. An eastward line from the airport to Botany Downs has also been proposed with a new interchange at Puhinui railway station, planned to be built in two stages, the first of which is said to be an early deliverable component of the Airport to Botany rapid transit line, planned to be operational by end of 2020 /early 2021. This encompasses a new at-grade bus/rail interchange and enhanced station. Buses will still use the existing local road to cross the railway line to/from Manukau, along with local traffic. The second phase provides a rapid transit overbridge across the railway line to provide a more direct and bespoke rapid transit connection. The new rapid transit link will integrate with the new interchange station on the overbridge. The first stage is estimated to cost $59 million to construct. The line will also go through Manukau railway station before ending in Botany.
