The addition of three more underground stations - including one which will eclipse even Britomart for passenger traffic beneath a full block of Albert St - will put far more of central Auckland within walking distance of rail than the existing terminus, which former mayor John Banks once derided as a "garage at the bottom of Queen St".
His predecessor, Christine Fletcher, pursued a pared-down redesign of a grandiose proposal she inherited from previous mayor Les Mills in 1998. He planned a $1.5 billion development dominated by 11-storey tower blocks that would have "obliterated" the surrounding heritage precinct and waterfront viewshafts.
Mrs Fletcher says she and Auckland City officials went to great pains to future-proof the project.
Britomart will in 2015 reach its hourly capacity of 20 trains entering the dead-end station through its 427m tunnel and then backing out. Mrs Fletcher says two outer sets of rail tracks have already been designed to push through the station's western end (see graphic).
Those were originally intended to form ramps to the surface for light rail tracks running out from Britomart to Queen St and beyond, but Mr Banks scrapped the plan after winning the mayoral chains in 2001.
Mrs Fletcher says their adaptation to a heavy rail underground extension is the only way to get trains circulating through the city to meet the Auckland Plan's ambitious public transport targets and stave off traffic gridlock through the CBD.
Auckland's former city council had to start digging the tunnel in 1999 as a covered trench in haste before developers began building over the route after a Treaty settlement deal between the Government and new land-owner Ngati Whatua, to avoid a considerable extra cost of boring underground later.
That meant the tunnel was ready well before the first train shuffled into the new station on July 7, 2003, marking the completion of a project which came in at $211 million, including $37 million for turning Auckland's former Chief Post Office into its grand public entrance.
Although it left the region's other 40 or so stations looking shabby by comparison, Aucklanders could at least take pride in hosting a world-class facility at the heart of their city which ultimately spurred a $1.6 billion upgrade of the rest of the rail network.
Mrs Fletcher says that as well as reviving rail, Britomart has been key to turning a once "scary" downtown Auckland into a more popular and liveable place, after design advice from the National Council of Women on how to make it safer for females.
"Britomart had a really bad aura to it - it was the B-word," she says.
"But now, every time I walk down there I feel an enormous sense of pride that Auckland under the right set of conditions can do the right thing.
"When I see those heritage buildings and those viewshafts out to the water, I think, 'we nailed it'."
The wider rail upgrade has included tens of millions of dollars of government and Auckland ratepayers' money pumped into new stations at urban growth centres such as New Lynn, Newmarket and Manukau and will see the first of 57 electric trains running in April at 10-minute frequencies on the southern line, before they are introduced to the rest of the network by 2016.
Although some claim Britomart is the world's only major underground railway station serving diesel trains, Mrs Fletcher says it was always intended to be "for electric units, not dirty old diesel, which we are shortly to get rid of".
She says it was no surprise to her when the Government changed its mind the week before last to support the underground railway plan, to allow trains to circulate freely through the CBD. "I have known, as John Banks discovered after he did the research himself and as the Prime Minister himself has discovered, that when we get down to evidence-based solutions, that is the only logical conclusion."
Mike Lee, chairman of Auckland Council's transport committee, says the decision to go electric was made in anticipation of running long and deep tunnels from the other end of Britomart as it would have been extremely difficult to design an adequate venting system for diesel fumes.
The left-leaning politician sees the Government's backing of the rail tunnels as a political masterstroke by Mr Key, who he understands had to work hard to convince "his political backwoodsmen, his senior ministers".
"That can only benefit National, but the certainty of it all has been really good for Auckland."
As for Britomart itself, Mr Lee says there is no doubt it pulled Auckland rail back from the brink of extinction but frets over a recent loss of momentum under stagnating patronage, which until last year more than quadrupled to just under 11 million annual passenger trips, from 2.25 million in 2002, but has since eased back to just over 10 million.
"It's not just a matter of building the big infrastructure projects - it's also about making sure the people who use the service are not priced out or turned away by poor service."
Although Auckland Transport chairman Lester Levy has promised "more attractive pricing" and greater service reliability, newly revised patronage projections by his board fall well short of meeting a challenge set by Mr Key to show enough customer demand to bring the construction start for the new tunnels forward from a Government date of 2020.
