A Metro train derailed with 63 people on board Monday morning outside Farragut North. Authorities said there were no injuries. (Alan Devlin)

A preliminary investigation shows a Metro train derailment on a downtown stretch of the Red Line Monday morning was likely caused by a broken rail, officials said.

The agency said a 7000-series train with 63 passengers aboard derailed as it passed over a section of defective tracks around 6:30 a.m. One passenger was evaluated for shortness of breath, but there were  no serious injuries, officials said.

Three railcars from the rear of the train came off the tracks, officials said, and the passengers were herded through the tunnel to the Metro Center station platform after an estimated 90-minute wait aboard the train outside Farragut North.

The early-morning derailment on a busy section of system in the downtown core raised new questions about the millions invested in the year-long SafeTrack maintenance program, and efforts to raise hundreds of millions more to improve the safety and reliability of the system. The cleanup, meanwhile, was likely to impact the Tuesday morning commute.

“This is a process that will take some time as you can imagine, and we’re going to be very methodical about it to prevent any further damage and obviously any safety-related issues,” Metro General Manager Paul J. Wiedefeld said. “As you can imagine it’s very tight quarters down there in the tunnel.”

Wiedefeld said “investigators are looking into whether a break in the rail developed as the train went over the incident,” particularly a “10-foot” long section of rail that “shattered in separate places” and is being rebuilt with different pieces.

“We saw some cracking, but again, it’s too early” to determine a root cause, he said.

Wiedefeld added, ” I can also tell you that this rail was manufactured in 1993, which may sound old but actually rail can last 40, 50 years just so you do know, so it’s not particularly old in the railroad business.”

Metro said the rail was last was subjected to ultrasonic testing on Aug. 9 and would have been visually inspected as recently as last week under its protocol.

The agency was in the process of pulling those inspection records on Monday.

“There were no issues identified during that [ultrasonic testing] process,” Wiedefeld said of the process that allows inspectors to detect cracks before they are visible to the naked eye. “I must emphasize that the final root cause has not been determined and we will continue looking at all possible contributing factors.”

Wiedefeld said the train slid up to 1,200 feet, kicking up dust and creating smoke clouds as it ground against the concrete.

The derailment happened about 6:30 a.m. after the train left Farragut North station. Passengers described a sudden jolt that felt like extreme airline turbulence.

Alan Devlin, a 37-year-old attorney who was headed into the office from his home in Cleveland Park, said as the train cruised through the tunnel, there was a loud bang followed by “unusual shuddering” and then a lurch. That was followed by smoke and a strong electrical smell.

“There’s a moment where you’re thinking ‘is this about to get worse or better?’” he said.

Red Line service continued to run Monday with delays. For a brief time, shuttle buses filled gaps in service. On Tuesday, Metro officials said trains are expected to run every 10 minutes between Glenmont and Shady Grove. Red Line trains generally run every eight minutes across the line with more frequent service from Grosvenor to Silver Spring.

Metro spokesman Dan Stessel said the 63 people on the train included 61 passengers, a Metro Police officer and the train operator. The number was small for a Monday commute, because the  government and many companies were closed for the Martin Luther King Jr. holiday.

The National Transportation Safety Board is monitoring the situation, but has not sent anyone to the scene, officials said. The Federal Transit Administration, which is responsible for safety oversight of Metro’s rail system until a new state-based agency takes over, said they have sent personnel to oversee the investigation.

The derailment comes just seven months after the conclusion of SafeTrack, the transit agency’s massive rebuilding program, which focused on repairing the system’s most degraded stretches of track. But the downtown core where the derailment occurred was not part of the unprecedented maintenance effort.

Monday’s derailment was Metro’s first on a passenger line since July 2016, when a train derailed outside East Falls Church station. No serious injuries were reported in that incident, but the station was closed for several days.

Aboard the train Monday, Devlin said the train’s emergency brakes engaged, and he and other riders held on to keep their balance. Then: “The whole carriage lifts and it yaws left,” he said. “The emergency brakes did their thing… it slowed down but not as quickly as you think.”

When it was over, Devlin said his railcar — the second from the end — was askew and raised six or seven above the others.

Around two minutes later, he said, the operator said: “We’ll be moving along shortly.”

“I remember laughing to myself, ‘I don’t think so,’” Devlin said.

Devlin said passengers were led to the front of the train, where they waited for about 90 minutes while crews debated whether to evacuate them onto a second train or lead them through the tunnels to Metro Center. Finally, authorities herded them through the tunnels to the platform, about 2,000 feet away. They were given glow sticks to guide their way.

“Like an underground nightclub,” Devlin said.

Radio communications from the Broadcastify website indicate that Monday’s incident was first investigated as a smoke event and that Metro’s control center had difficulty communicating with the train operator. Breakdowns in radio communications were among the problems cited in the investigation into fatal 2015 incident at L’Enfant Plaza, in which Carol Glover, a 61-year-old grandmother from Alexandria died.

The first radio transmissions began around 6:30 a.m. with the report of smoke between Farragut North and Metro Center.

“Smoke observed coming from train 106 at Farragut North,” a dispatcher said.

Metro’s rail operations  control center was then asked to investigate the report.

“I got as far as the smoke, all I see is smoke in the tunnels,” an employee is heard saying. “It’s no visibility,” says an employee in another exchange.

Authorities gave passengers glow sticks to guide them through the tunnel after the derailment Monday morning. (Alan Devlin)

The ventilation fans were turned on, but it appeared radio transmission problems prevented further communication with the train. A dispatcher repeatedly asked for updates from the train operator.

“106 can we get an update, location, good radio communication? Over.”

“Train 106 track 1, can we get an update? . . . Get you to a location where we can get a better radio communication?”

Then, an update from outside the train: “Officer was on the train, he said it seems like a derailment — he said just all of a sudden smoke came out of nowhere, he thinks it derailed but I’m not too for sure,” an employee said.

Finally, a man who may have been the Metro Transit Police officer aboard the train contacted central control.

“It seems like we probably derailed leaving Farragut North station,” he said. “We are obviously stuck in the tunnel, there’s no radio transmission on my radio or phone reception so what do you need from us and what’s the plan?”

Wiedefeld acknowledged some “spottiness” in Metro’s radios.

Doug Buchanan, the D.C. Fire and EMS spokesman, said the fire department’s initial call was for a report of smoke. The fire department cleared the scene about 9 a.m.

Stessel said the train operator is cooperating with investigators and will be subject to standard post-incident alcohol and drug screenings. He could not say how much experience the operator had, only that it was a male operator.

He said derailments typically focus on three things: human factors, including how the train was operated; track and infrastructure issues, including the condition of the track and whether temperature fluctuations were a factor; and rail car-related factors.

The derailment, on a quiet holiday weekday, came as Metro touted a bolstered safety and reliability record in the wake the year-long SafeTrack program and an initiative known as “Back2Good,” aimed at winning back riders.

Though there were no serious injuries Monday, the incident could further complicate Wiedefeld’s efforts to secure $15.5 billion — including $500 million in permanent annual funding — to ensure the system’s safety and reliability.

Metro Board Chairman Jack Evans, who was briefed by Wiedefeld on Monday morning, was shocked by the news.

“It’s like, God, didn’t we do all of the fixing, the bad areas, SafeTrack?” he said. “All that stuff was intended to prevent stuff like this from happening.”

The holiday also posed a logistical challenge because the Metro personnel needed were scattered around the region, and some were out of town, Evans said.

“It is puzzling to me,” he said. “My first reaction was, ‘What?’ You know. How did we have a derailment? How do we have derailments? And particularly the Red Line in the city is something we’ve done a lot of work on.”

Later in the afternoon, he applauded officials on their quick response to the incident, and renewed his call for dedicated funding.

“Overall in looking at this, a bad day when anything goes wrong, but a good day in how we responded to it, and it shows you the progress again that Metro has made,” he said. “It also feeds into what I’ve been saying for the last three years. Metro needs the dedicated funding of resources to continue to fix our system. And so I reach out to all of the jurisdictions, Maryland, D.C. and Virginia to move as expeditiously as you can in getting the resources to our system.”

The incident came three days after third anniversary of the fatal L’Enfant Plaza smoke incident that killed one person and sickened scores of others after a Yellow Line train became stuck in a tunnel and was engulfed in smoke.

Metro’s last derailment, in July 2016, was caused by a condition known as “wide gauge” where the tracks spread too far apart to support a train. The NTSB later concluded that Metro knew of hazardous conditions in the area more than a year before the derailment. As a result of that incident, the agency fired a third of its track inspection department last year after a sweeping investigation found a pattern of records falsification among track inspectors.

About a year earlier, a train derailed at Smithsonian Metro station during a busy morning commute. Metro said that the same track defect —  wide gauge — had been detected more than a month before along the stretch of track but was not repaired.

Rachel Weiner contributed to this report.