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Rail officials have begun planning for a $1 billion central Auckland "loop" tunnel considered the key to ending growing congestion on tracks into the Britomart station.

The Auckland Regional Transport Authority revealed last night that it had begun preliminary planning for a 3.5km tunnel between Britomart and Mt Eden, beneath Albert St and including underground stations near Wellesley St and Karangahape Rd.

Spokeswoman Sharon Hunter said the authority was working in conjunction with the Auckland City Council to "protect" the route, although she did not have details of how that might be achieved, and whether any restrictions may be looming on conflicting property developments.

Although the authority proposed the idea of tunnel as part of an inner-city rail loop to its Auckland Regional Council parent in 2005, Ms Hunter said her board had only recently decided to put the planning wheels in motion.

A Treasury-led group of officials assessing Auckland's transport needs suggested last year that an early start was needed on building a case for the tunnel.

This was because because of a likely 12 to 16-year lead-time for such a project, and an expectation that Britomart's capacity would become "critical" by about 2020,

Ms Hunter said a rail loop would make it possible for 400,000 Aucklanders to reach the central business district within 30 minutes by rail, doubling the existing catchment.

By turning Britomart into a through station, the project would allow greater frequencies than the 10-minute services expected by the end of next year from the Government's $600 million upgrade of Auckland's basic rail network, much of which involves duplicating the western line.

The authority's disclosure follows criticism from commuters sick of having to wait in trains outside the Britomart rail tunnel for other services to pull out and make space for them.

The station has five platforms, but the 9.3m width of its 426m tunnel allows only one line in and one out.

Ms Hunter said shorter-term solutions included better network reliability, with a focus by the Government rail agency, Ontrack, on improvements to signalling, and negotiations for a higher level of maintenance by Toll Holdings of the region's elderly commuter trains.

Full duplication of the western line should also allow better sequencing of arrivals and departures at Britomart.

Pressure on the terminal will increase when branch lines from Onehunga and the Manukau City centre open by the end of next year.

But Ms Hunter said Ontrack's rail upgrade included optimising tracks and signals between the Quay Park junction and Britomart.

Trains will run along 1.1km of new tracks through New Lynn from next week, to clear the way for a $100 million-plus railway trench.

Passengers will use a temporary platform to reach the tracks from Monday, although from the same direction by which they now gain access to New Lynn Station.

Ontrack is looking forward to a busy weekend connecting signals on the new tracks to the rest of its network, during which the transport authority has organised bus shuttles to replace trains west of Avondale.

Traffic barrier arms and pedestrian "mazes" will be moved to cover new tracks at three level crossings, on the Clark St-Rankin Ave roundabout, Veronica St and Portage Rd.

Project manager Peter King said the tracks were to give contractors enough safe operating room for heavy cranes to drive piles up to 40m deep for the first of two 800m retaining walls between Whau River and the western end of New Lynn.

First to be built would be the northern wall, along which trains would run while the southern structure was constructed.

A trench would then be dug between the walls, 20m wide and up to 8m deep, for duplicate tracks and a permanent below-ground station.

Mr King said a new pedestrian crossing had been built over the railway tracks from Clark St.

The trench is aimed at easing traffic congestion through New Lynn by enabling trains to run below road level as their frequency increases.