To nobody's surprise, an Auckland underground rail scheme has failed a cost-benefit analysis for Transport Minister Steven Joyce. A review by the Ministry of Transport, the Treasury and the NZ Transport Agency has calculated the likely benefits to be worth no more than 40 per cent of the $2.4 billion it would cost for a tunnel with three stations between Britomart and Mt Eden to complete an inner city loop.

Mayor Len Brown was not surprised. His council saw the verdict coming weeks ago and had a rival evaluation made by its consultants PricewaterhouseCoopers, Beca and others. Their report, dated May 13, was released by the mayor hard on the heels of Mr Joyce's announcement on Tuesday. PWC, Beca and the others have calculated the benefits to exceed the costs by a ratio ranging from 1.1 to 2.3.

The public is left to wonder how two panels of experts could come to such wildly different conclusions, and we can but ask ourselves, who can we believe?

Perhaps the most useful explanation for the divergence has come from a PWC director, Chris Money, once a senior official in the Transport Ministry. He said the officials' report had worked from past trends to predict the effect on the city whereas the report he helped to prepare had focused on the region's plan for greater urban intensification.

That means, the council's consultants have assumed rather a lot. The region's plan for greater urban intensification depends on the rail loop. The planners are confident that if Auckland can be equipped with fast, frequent, reliable trains, the living and travelling habits of its citizens will change. More of us will want to live in higher density units close to train stations and work in the central city.

Well, maybe. It is not a lifestyle that has attracted much interest when previous councils have tried to encourage it, but maybe a central city loop would make a difference. If trains can run through Britomart rather than terminating there, more services can be provided.

The proposed underground line would have stations under Albert St, Pitt St and Upper Symonds St, linking them with the surface line that already provides stops at Mt Eden, Grafton, Newmarket and Parnell. The circuit sounds attractive but not enough to pay its way. The project would add $18 million a year to the operating costs of Auckland's passenger rail services.

While Mr Joyce's officials are sceptical of the predicted benefits they have agreed there is "a strategic case" for the council to designate the route. This small, protective step is about all the council can take for the moment and the mayor will use it to give the project an appearance of momentum. But it is no closer to getting finance from the Government.

The officials conclude, "The central city rail loop does not justify further consideration for central government funding at this point in time because the project does not represent an economically effective investment."

Undaunted, Mr Brown says the council has committed $2 million from the ratepayers for investigations and he seems confident it will put up more money for buying property along the 3.5km route. Mr Joyce says the Government has no objection, "It's their money".

It appears national transport planners would have no objection to Auckland financing the whole project if the council could convince its ratepayers to carry the debt and operating costs. That would be the most reliable test. No matter how exciting an underground line sounds, no matter how essential it may be to the mayor's "vision", would we pay for it?

Aucklanders readily agreed to finance a harbour bridge because they knew they would use it. Underground rail is a bigger gamble but it is one a Super City can take.