Few believe the city will complete Oahu rail to Ala Moana Center for the $9.2 billion total it projects, and we hear much speculation about the real final construction tab. $10 billion? $12 billion? $15 billion?

We’ll never know the answer if the Honolulu Authority for Rapid Transportation enters a planned public- private partnership (P3) to build the last four miles down Dillingham Boulevard and through the city center.

In its recovery plan recently approved by the Federal Transit Administration, HART sold P3 as a way to control construction costs after going $4 billion over budget in earlier segments.

But the practical effect will be to conceal the true cost of building the final leg, and it’ll almost certainly inflate future costs to operate rail — already estimated as high as $140 million a year, with no clearly identified way to pay for it.

The Legislature’s last bailout left HART only $1.4 billion to build the final four miles plus a major parking structure at Pearl Highlands. Legislators and the FTA have said they’ll give no more.

If HART really thought it could finish for this much, it would go with a straight construction contract and expect a winning bid of $1.4 billion or less.

But city leaders know the tricky leg through the city center couldn’t likely be built for that amount after massive overruns on earlier segments built much of the way through open fields.

Enter P3, in which private partners agree to finance and build the remaining line for no more than $1.4 billion in exchange for lucrative rights to manage rail operations for 30 years.

Knowing they can’t actually finish the construction for $1.4 billion, private partners will figure how much they’ll really have to front and pad the operating component of their bid to make up the difference, with a generous cushion, interest and some “sweetener” for their trouble.

One possible sweetener is development rights along the rail line, meaning those valuable lands could be used for more of the profitable high-end condos, hotels and retail that developers want instead of the affordable housing we need.

Complicating matters is that Ansaldo Honolulu JV, builder of the rail cars, already has operating rights for nearly half the 30 years and would have to be paid off to cooperate.

The result is we will once again pass on the bill to our children and grandchildren and never know the true cost of either construction or operations.

Which makes it tough to buy Mayor Kirk Caldwell’s recent assertion that, “Counter to what opponents of the rail project continue to claim, the city remains on solid financial footing.”

It tells us all we need to know about rail’s troubles that he considers those who advocate honest, responsible and straightforward financing to be opponents.

Reach David Shapiro at volcanicash@gmail.com.