Honolulu rail could soon face another court challenge, even as it grapples with financial trouble.

A new group of some 20 local residents dubbed “Do Rail Right” says it’s concerned that long stretches of the multibillion-dollar transit project are being built on Oahu’s flood plain and could be vulnerable to damage from tsunamis, storm surges and sea-level rise linked to climate change.

The group asserts that rail officials haven’t done enough to address those risks or to comply with federal flood-risk standards. If the Honolulu Authority for Rapid Transportation, which oversees the rail project, doesn’t agree to revisit its previous studies to address the flood issues, members of Do Rail Right say they’re prepared to take the rail agency to federal court.

“These failures … are shocking,” Do Rail Right attorney John Carroll, a former state senator, wrote in a letter Friday to HART board Chairwoman Colleen Hanabusa. On Monday, Carroll said he plans to send Hanabusa another letter today giving HART seven days to agree to address the flooding concerns with a new supplemental environmental impact statement.

HART spokesman Bill Brennan said in an email Monday that the rail agency has fully complied with federal law and the National Flood Insurance Program.

“The Project’s Final EIS did consider and evaluate elements of the Project that are in the floodplain,” Brennan wrote. The project will comply with the National Flood Insurance Program via “applicable permits and approvals,” he added.

The group contends that if HART needs to produce a supplemental EIS on the flood threats, it will inevitably force the rail agency to move the elevated transit line’s route from the low-lying coastal areas between Pearl City and Kakaako to inland mauka areas, according to spokesman Al Frenzel.

Frenzel suggested that officials could then revisit running the rail through Salt Lake as previously envisioned, instead of by the airport.

Members of what eventually became Do Rail Right have been meeting every weekend for the last couple of months, said Carroll, who is representing the group for free.

Members say they don’t aim to stop the project, but rather save it from “infrastructure damage,” as Frenzel put it. Even though the rail line will be elevated, its elevators, escalators, power sources and parking facilities would be underwater, they say.

The group has emerged as rail leaders are hoping a recent five-year rail tax extension will generate enough dollars to finish the estimated $6.57 billion project.

Do Rail Right further argues that the city’s recent expansion of its tsunami evacuation zone maps, along with a 2015 executive order signed by President Barack Obama aimed at taking more action against flood risks, calls for HART to revisit the issue. The group also points to the University of Hawaii’s research on how sea-level rise will likely affect Oahu’s South Shore.

Brennan said that Obama’s order is not retroactive, so it doesn’t apply to the project’s past approvals. Approvals for any parts of the project in the flood plain will comply with the most recent maps and regulations under the federal government’s flood insurance program, he added.