Rail board members will hold a special public meeting today to discuss the cash-strapped transit project’s ongoing costly challenges with overhead utility line clearances — a problem that an independent oversight firm has dubbed rail’s “most significant risk.”

The meeting, held by the Honolulu Authority for Rapid Transportation board’s Project Oversight Committee, will take place 5:30 p.m. at Mission Memorial, 550 South St., next to Honolulu Hale. It occurs about a week after federal officials confirmed that they estimate it could cost more than $8 billion to finish the entire 20-mile line to Ala Moana Center, throwing the future scope and design of the project into question.

Meanwhile, questions remain over how the safety clearances for Hawaiian Electric Co. to repair and maintain its transmission lines hanging above the rail guideway suddenly became such a big and expensive challenge to the project, rather than something that rail and HECO officials resolved years before building began.

HART and HECO officials have offered differing accounts of how exactly the issue developed over the years. Both parties have been slow to respond to requests for more details and have offered only partial answers in recent weeks.

HART has said that it and its predecessors at the city handling the project were kept in the dark about HECO’s safety clearance requirements until 2013. The rail agency has since resolved that it must place some 1.8 miles of electrical lines underground along Dillingham Boulevard, as well as nine poles near the airport — and it’s still working to resolve clearance issues along the rest of the line.

The lines that must go underground along Dillingham Boulevard include 138-kilovolt lines, which officials say will be particularly costly.

However, an October 2011 report by rail’s federal oversight contractor, Jacobs Engineering, in consultation with the city, compiled a table of all the potential known risks to the project at that time. Those risks in 2011 included the “relocation of the 138 kV overhead power lines” and how the relocation might require additional lines to supply power during that time.

In an email provided earlier this month by a HART spokesman, the rail agency distinguished general “relocation” of the lines from the specific need to put them underground. The relocation could involve other, less costly actions, such as placing the lines higher on the utility pole or even up against the rail guideway, as HART had previously proposed, according to spokesman Bill Brennan.

Still, it remains unclear how it is that the firm overseeing rail mentioned the relocation of the 138-kV lines some two years before rail officials say that HECO alerted them of the clearance requirements. HART has still not responded to requests for clarification.

HECO, meanwhile, has said that it corresponded with HART and its contractors in writing “dozens” of times between 2009 and 2012 on the clearances issue, when HART said it was in the dark.

On Monday HECO provided a packet of correspondence between 2009 and 2012, including a letter to rail contractor MK Engineers and 10 “review comments” to Kiewit Infrastructure West on the need to maintain proper clearances. Kiewit is building rail’s first half on Oahu’s west side.

None of the correspondence provided Monday was addressed to the city or to HART during the time HART said it was kept in the dark. Darren Pai, a HECO spokesman, said the utility company has other correspondence but that it is subject to nondisclosure agreements.

Typically, the HART Project Oversight Committee, which does not include the entire board, meets just before a regular full board meeting.