Four years ago, a routine Metro inspection turned up something unusual: The distance between wheels on one of its new rail cars had widened. The transit agency viewed it as a straightforward warranty repair issue.

A second case was detected later that year. In 2018, two more were found, doubling to four the following year. Last year, five wheelsets showed the problem, which could hinder a train’s performance and possibly force it off the track. Despite the increase, the number of wheel-assembly problems was small, considering the several thousand inspections Metro conducts annually on its 1,200-car fleet.

This year, the number rose sharply and began to raise flags. Metro inspectors had found 18 cases before the issue jumped from Metro’s rail yards and repair shops, into public view with the Oct. 12 derailment of Blue Line train No. 407. A National Transportation Safety Board investigation quickly zeroed in on the widening wheelset of the fourth axle on rail car No. 7200. It prompted an emergency examination of every 7000-series car that turned up at least 21 more cases.

Metro had been on a slow rise 20 months after the pandemic nearly brought it — and the Washington region — to a standstill. But as the health emergency was slowly subsiding and more riders trickled in through fare gates, a second quagmire socked the nation’s third-largest transit system at a time when it needs riders the most. The rare defect has morphed into Metro’s biggest crisis since a deadly smoke incident six years ago.

The defect’s acceleration among 7000-series cars led the independent Washington Metrorail Safety Commission to order all 784 cars from service — grounding about 60 percent of Metro’s fleet — forcing the transit system to scale back service at a critical juncture in the region’s recovery.

Much remains under investigation, but the puzzling safety failure has led to one overarching question: Why didn’t Metro act sooner? Interviews in the days after the derailment with top Metro leaders, the head of the safety commission, and current and former Metro contractors and mechanics provide a glimpse into the circumstances that could have kept the problem from being reported to the safety commission, as required, and from being revealed to Metro’s board of directors.

Mechanics say the rarity of the defect — it was found in 4 percent of cars over four years — may have led those on the front lines to believe the issue was an outlier. Metro’s long-standing problems with record-keeping and documentation of repairs, which were cited by the safety commission one month before the derailment, are also part of the investigation.

An advanced rail car’s ‘progressive defect’

Metro’s 7000 series cars were a breakthrough in design for the rail system. They were the seventh model of Metro cars, ushering in a dramatic change over the previous six.

They were gleaming silver, not tan and brown, and their interiors were spacious with wider seats, aisles and entryways that allowed for better flow of passengers. They also had the ability to create eight-car train sets instead of six, boosting capacity on the rail system.

“The design has been improved based on our thirty years experience in operating and maintaining these vehicles,” Metro officials wrote in a 2008 staff report, seven years before the first cars were delivered. “While it is, in many ways, completely familiar, the intent has been to keep the elements that work well and, with those that do not, replace them with proven components.”

Metro had never ordered more than 300 cars in a series. In this case, the transit agency ordered 748 cars from Kawasaki Rail Car, each costing about $2 million, adding them to the fleet between 2015 and 2020.

In 2018, wire crimping defects required the cars to be rewired, but otherwise, the 7000 series was seen as a success within Metro, helping to propel a dismal on-time performance to 80 percent about one year into their introduction. A performance report in 2019 showed the cars had helped on-time performance jump up to 89 percent.

The cars broke down less often than previous series, which is a reason the sudden spike in wheel assembly failures this year surprised investigators and provided them with a clue that problems were likely related to the cars’ construction or assembly. Investigators have no indication the problem originated during Metro’s maintenance or repairs of the cars, said David L. Mayer, chief executive of the safety commission, which is part of the NTSB investigation. Congress created the commission in 2017 to oversee Metro safety.

Mayer called the problem a “progressive defect” that surfaces over time.

“In common language, a defect usually means you took something out of the box and it was broken before you got to use it at all. And that’s not what we’re talking about here,” Mayer said. “It’s a progressive defect, in that it appears and it gets worse over time or over miles, travels or other usage metrics.”

It’s not a problem Mayer has seen in his years as managing director of the NTSB and chief safety officer of New York’s Metropolitan Transportation Authority.

While Metro also pulled its 6000-series cars from service after two train separations last year, Metro General Manager Paul J. Wiedefeld said Friday no analogies should be drawn between the two sets of repairs. The 6000 series, which are slowly returning to service, have been out-of-service since November. Their repair involved purchasing new equipment to rebuild the apparatus that joins rail cars, Wiedefeld said, insinuating that the 7000 series’ repairs appear to be less intensive.

Metro has not given a timetable for when the 7000 series could return, but officials say they are trying to return the cars as soon as possible — possibly without fully knowing the cause of the defect, but adding more frequent inspections.

“There may be some unique circumstances that are at play here that Metro needs to get a handle on, but in the end, these are rail cars, axles and wheels like [others that] operate all over the country and all over the world,” Mayer said. “There has to be a way to operate them safely, so I’m very optimistic about that.”

A little-known problem in Metro rail yards

The idea that wheels had been shifting on their axles came as a surprise to rail industry experts and longtime Metro employees.

After being told by superiors that cars needed back-to-back inspections to measure the distance between the wheels, one Metro mechanic said he didn’t initially understand why because he wasn’t familiar with the defect. The mechanic said Metro’s engineers were still speculating as to the underlying cause of the problem.

“There’s a lot of unanswered questions,” said the mechanic, who spoke on the condition of anonymity because he is not authorized to speak to the media. “The engineers are scratching their heads.”

The mechanic said Metro also has been inspecting bearings on the cars. The NTSB confirmed that was part of its investigation, but declined to elaborate.

A Metro engineer, who spoke on the condition of anonymity because he was not authorized to speak to the media, said he also was not familiar with the issue of wheels shifting on axles until this month. He cautioned that the investigation was still in its early stages.

Lemuel Proctor, who started at Metro in the 1970s as an electrical mechanic and rose to become its chief operating officer for rail, said in a 30-year career he never encountered a wheel that moved on its axle.

“Anything to do with those wheels and axles, and the trucks, period, is serious,” said Proctor, who retired from Metro 2004. “That’s what keeps you on the track.”

Trains roll along the rails with a flange at the edge of the wheels to keep them on the rails. The distance between the wheels has to be within a tiny fraction of an inch. The wheels are pressed onto axles at incredibly high pressure to ensure they stay fixed in place.

On Friday, news site Greater Greater Washington reported that Kawasaki Rail Car had initially been pressing the wheels onto axles of 7000 series cars at lower pressures than was typical for other Metro cars. The pressure was increased in 2017, but it was not clear whether cars that already had been manufactured were altered.

Kawasaki has not responded to numerous requests for comment.

In the case of the derailed Blue Line train, the NTSB said the wheels had shifted outward, and passing over a switch bumped the train off the rails. It was the rail car’s third derailment of the day, but passing over a second switch had bumped it back onto the rails on the first two occasions, investigators said.

Oversight commission left in the dark

The question as to who knew about the defects within Metro and why it wasn’t reported to the commission that has safety oversight is part of the NTSB’s probe. The safety commission, which began operating in 2019, has said it did not know about the wheel assembly problems.

Before the commission, the Federal Transit Administration had oversight of Metrorail safety. The FTA declined to comment on whether Metro informed federal officials of the defects in 2017 or 2018. Had the FTA known about the issue, it would have been significant enough for federal transportation officials to have briefed the safety commission on the matter, Mayer said, adding that the FTA never mentioned it.

Metro Board Chairman Paul C. Smedberg said the board also wasn’t aware of the issue. While it remains unclear how far up the chain of command the discoveries of widening wheel sets rose, it likely went above those conducting rail car inspections.

According to federal officials briefed on the investigation who weren’t authorized to speak publicly, Metro asked Kawasaki in 2017 — the year the problem was discovered — to perform a failure analysis on the defects, but such a review never happened.

Metro tried to address the issue by increasing the pressure used to push wheels onto axles after more than 490 of the 748 cars were delivered, officials said. Still, the defects appeared in cars after the modifications were made.

Investigators want to know if Metro adequately tracked the defects.

According to a safety commission audit released in September on rail car inspections, maintenance and best practices, Metro has been repeatedly cited for doing repairs and overhauls of rail cars without a proper system of quality checks. It also inadequately documented repairs and maintenance, according to the audit, which the commission said has contributed to safety failures.

“Metrorail does not have adequate document control practices for car maintenance job plans, does not clearly define the use of certain engineering documents, and does not have a systematic process to ensure that mechanics and engineers are trained for the specific tasks they are assigned to perform,” the audit said.

The transit agency responded to the audit last month by saying it has created more organized systems to keep better records and was “committed to continuous improvement of our programs and enhancing the safety of the system.”