The financially handicapped rail project is at a critical juncture, its planners awaiting a signal. Either the city will be directed to continue construction to its completion, or that door will slam shut.

That’s the dark cloud that a roughly $3 billion shortfall has cast on the project, now approaching the $10 billion mark in anticipated costs. In order to lift that cloud, some innovative financial models will be required, and one example has emerged in the form of a public-private partnership plan.

This should be carefully studied by the city, which also is hammering at the door of the Legislature in hopes of improving its prospect for public funding.

By the end of April, the city and the semiautonomous Honolulu Authority for Rapid Transportation must provide a “recovery plan” for the Federal Transit Administration, a new financial blueprint showing how they intend to get to the finish line.

Into this crisis steps Ulupono Initiative, a social investment fund headquartered in Honolulu. The initiative looks for and invests in enterprises that support Hawaii’s sustainability.

And, its organizers said, the fact that rail will be powered by electricity rather than fossil fuel fits into its mission.

Ulupono commissioned a study by Jones Lang LaSalle (JLL), an infrastructure finance specialist. Its job was to explore alternative financing schemes. To this point, rail’s primary funding source is a half-percentage-point surcharge on the state’s general excise tax. Authorized through 2027, the tax is being proposed for further extension by city and rail officials to help cover costs.

The JLL researchers did not recommend this extension particularly but it did observe that a purely public funding source is essential. These could include the extension as well as other taxes to be assessed at the city level, according to the report.

The sum of the report is a recommendation for public-private partnership as a way to draw investor cash to the project.

This approach is also called “design-build-finance,” a scheme that offers incentives — generally, some level of gains on what’s invested. But to make those gains — to be paid at all — the builder must deliver the project.

This, its advocates said, will help drive timely completion of the work, a prospect that is attractive to taxpayers.

The GET surcharge remains the most broad-based and logical way to finance rail because it would draw on visitors to the state as well as residents, all of them likely to use the rail.

Even so, the city must contemplate a variety of options. State lawmakers, including key legislators who are leery of extending the GET surcharge, may be persuaded more easily if they see city officials would not rely solely on the state-authorized tax.

The question is whether such an investor-designer-builder team could be assembled that is equal to the task. Companies often balk at assuming risk up front.

The city should point to other municipalities such as Vancouver that have succeeded with this model of financing a transit project. Payment comes from a stable taxpayer stream. But unlike bonds, this is not a loan and does not add to the municipal debt; it’s a delayed payment to an investor-builder who is highly motivated to finish on time.

Krishniah Murthy, the acting HART executive director, said this is no silver bullet. The remaining section of rail left to build may not present enough profit motivation, he said.

Nevertheless, it could be part of a package that is anchored by the sensible extension of the bedrock GET surcharge.

If the city wants to realize the benefits of a fully developed rail system, one that spans the distance from Kapolei to Ala Moana, the components of its updated financial plan has to start coming into place. Some outside-the-box thinking certainly couldn’t hurt.