The development of Honolulu’s rail-transit system is a saga, but it should not be quite the mystery it’s been.

The unanswered question: Why are cost estimates so out of whack?

Why have the projected costs doubled over the years, with the bottom line now seeming closer to the $8.1 billion forecast by federal authorities?

The feds, long seen as immovable on the central specifications of the project, are worried enough about cost control that they’re willing to hear proposals from the agency at the helm of the construction, the Honolulu Authority for Rapid Transportation (HART).

HART was given until Aug. 7 to inform federal transit officials whether or not it plans to change tracks.

But in order to make the right decision in any quest for cost savings, the agency will need a better way to project expenses than what’s been used to date, given the double-digit increases the public has witnessed.

City officials must use their power — affirmed through the new ordinance that extended the project’s financing tax surcharge — to keep a watchful eye over how the money is being spent.

And the candidates vying to become mayor for the critical next four years must specify what they intend to do to direct the project toward a better course. Sending and hewing to a message that costs will be examined very closely would set the right tone.

HART also must ride herd over contractors and subcontractors to rein in costs as much as possible.

It’s likely that the authority’s best option will be to keep the current Kapolei- to-Ala Moana alignment and at least most of the station plans intact. But six alternatives were being contemplated last week:

>> Complete current plans to Middle Street, but then construct only the guideway to Ala Moana Center, eliminating all seven of the stations envisioned in between. But no public purpose would be served by passing over seven of the destinations where many riders would likely get off.

>> End the rail at Middle Street, with passengers transferring to complete the journey via bus line. HART would need to determine how much this would suppress ridership from commuters who don’t want a two-step trip.

>> Continue the alignment from Middle Street to Ala Moana Center with an at-grade rail system. Costing out the more expansive excavation would involve more complexity, and time. New burial sites might be found and require vetting by the Oahu Island Burial Council. And the work would take up even more ground space and intensify disruption to businesses.

>> Build the project as far as funding allows, and consider cutting or deferring stations to help lengthen the line. This “winging it” approach abandons good planning principles. The terminus point does matter.

>> Entertain public-private partnerships in which investors could help pay some construction costs. This is the one notion that seems sensible, but it’s yet unclear what appetite the business community has for developing around stations.

>> Change the Dillingham Boulevard alignment to Nimitz Highway — except that most desirable residential and business destinations are a long walk away, several blocks mauka.

HART officials weren’t pleased with any of them, for the reasons described, among others. None would complete the line to Ala Moana for the $6.8 billion the project is now set to receive from current taxing authorization.

More delays would result in order to do the additional studies for new alignments and complete the additional engineering work such plan changes would require.

There have already been shocking discoveries of changes needed, such as drilling far more deeply into the ground for support posts along Dillingham Boulevard and allowance for more space in construction around high-voltage power lines.

Colleen Hanabusa and others on the board asked HART staff to explain why the numbers have been so far off, and so far, no acceptable answer has been tendered.

It’s not good enough to say that the project has been dogged by “bad luck,” as Executive Director Dan Grabauskas has said. He also has said that it’s common practice for general contractors to handle the finer cost details.

Simply put, Honolulu’s project faces additional challenges and needs additional oversight.

Fortunately, that’s now required. Bill 15, enacted in February, mandates that HART report on such details to the Council within 30 days of the end of each fiscal year.

Public input could be useful to find the way forward, and the people’s elected representatives ultimately may need to re-evaluate the allotted tax revenue. But the decision on the right course is HART’s to make and should not be left, as one board member suggested, to a public poll.

The people voted for a 20-mile system, along a carefully studied alignment. A system that follows that basic course and provides sensible commuter connections is what the people said they want — and still need.

It’s HART’s job to deliver something as close as possible to what was envisioned, and promised.