The Jan. 15 derailment of a Metro Red Line train was caused by rust on the underside of a steel rail that was not visible to inspectors or detectable with ultrasonic scanning technology, according to findings from an outside consultant that were revealed Thursday at a meeting of the transit agency’s board.

Safety officials for Metro also announced that they are making permanent changes to the track inspection protocol as a result of revelations, first reported by The Washington Post last week, that defects flagged by federal inspectors as urgent “code black” issues were routinely reclassified by Metro as lesser priorities.

Chief Safety Officer Patrick Lavin said the downgrading of defects resulted from terminology that was “confusing.” Officials are making eleventh-hour changes to a soon-to-be-released track inspection manual, which he said will clear up the disparity between federal inspection reports and Metro’s own findings.

Both the derailment and track defect issues were raised at Thursday’s meeting as Lavin sought to assure board members that safety at Metro is on an upward trajectory. No one was injured in the January derailment outside the Farragut North station, but a new 7000-series train was damaged.

Lavin said outside experts confirmed preliminary findings that a 25-year-old piece of steel rail snapped under the weight of a train because of rust that had developed on the underside of the rail.

The rust caused small perforations known as corrosion pitting, which eventually formed a crack that snapped under the weight of a passing train. The damage to the rail was perhaps exacerbated by fluctuating temperatures in the days leading up to the Jan. 15 incident.

Metro officials said outside consultants concluded there was not much that could have been done to identify the rust problem in advance.

“The report states that this type of failure would have been undetectable with conventional ultrasonic testing, as it will not pick up a corrosion pit in the base of the rail,” Lavin said.

He said the report’s author was “not aware of any industry-accepted inspection technology that can detect a corrosion pit at the base of the rail.”

“It’s very, very rare. It’s not unique to this system,” added ­Metro General Manager Paul J. Wiedefeld. “Everything we need to do is just continue to do the testing that we’re doing.”

Officials said they plan to release the full report from the outside experts later this month.

“It’s always troubling that no matter how much effort we put in, there are things that can rise to the surface,” Metro Board Chairman Jack Evans said after the meeting. “But I’m confident that we can take care of these things.”

Lavin said the agency is making one permanent change in the after­math of the derailment: Metro will alter its evacuation procedures to encourage staff and first responders to evacuate passengers by foot, rather than wasting time trying to bring a rescue train inside the tunnel.

The agency was criticized when it took 90 minutes to guide passengers off the train and escort them to a nearby station.

If Metro staff and first responders had initially opted to evacuate on foot, Lavin said, riders would have gotten out of the tunnel 45 to 60 minutes earlier.

Agency officials also briefed the board on recent revelations that Metro staff were ignoring or holding off on repairs of track defects that were flagged by federal inspectors as code black — the most serious kind of deficiency, which usually requires tracks be immediately pulled out of service for repairs.

A Post examination of inspection reports from the Federal Transit Administration, which has safety oversight of the rail system, found its inspectors had flagged dozens of code-black defects — in some cases flagging the same defect multiple times over the course of months — that Metro did not immediately remove from service for repairs. In fact, in most of the cases, Metro inspectors downgraded the severity of the defects.

Lavin told the board that Metro “does not refute” the conclusions made by federal inspectors, and insisted that “we will not hesitate” to take a track out of service in the case of a safety-critical track problem.

Rather, Lavin said, the reclassified code-black defects resulted from a problem of semantics: Metro’s track walkers and federal inspectors have been using the label “black” to refer to two different types of high-level defects — urgent safety-critical problems and routine maintenance priorities.

Lavin compared the differences to problems on a car. A flat tire and a sizable dent in a car door might both be deemed urgent issues for repair, he said, but only one requires that a car immediately be taken out of service.

“While the [Metro inspection manual] makes a distinction between a safety and maintenance defect, the color black can be associated with both,” Lavin said.

“I agree,” he ceded, “that is confusing.”

As a solution, Metro will use different colors to distinguish between problems that should be bumped up on the priority list for scheduled maintenance, and problems that require tracks to be immediately taken out of service.

Robert Lauby, chairman of the board’s safety committee, said he has recently spoken with FTA officials who said there was “no disagreement on the fact that [defects] were appropriately classified.”

“FTA doesn’t have any issue with the way WMATA has classified these defects,” said Lauby, who is chief safety officer at the Federal Railroad Administration. “Both [Metro] and FTA seem to agree that they’re taking appropriate steps.

“Although,” Lauby added, “on paper, it’s confusing.”

In The Post’s March 3 article, the FTA said it stood by its inspections and the conclusions of its investigators.

“FTA and [Washington Metropolitan Area Transit Authority] inspectors use the same definitions of track conditions, as found in the WMATA manual. WMATA then conducts follow-up inspections to determine the appropriate response, which they have some discretion to decide,” the FTA spokesman said. “However, those follow-up actions do not change the original finding.”