During the investigation, the National Transportation Safety Board said it would not speculate on the cause of the accident. Investigators are currently inspecting the crime scene and examining the train cars and equipment involved, a spokesman said.

When rail safety expert Keith Millhouse looked at images and videos from the scene, he said he noticed the lead car had veered “significantly” off the track. “That suggests to me that there may have been excessive speed or that the train, after it derailed, still had a drive that kept it moving until it stopped,” said Millhouse, a California-based transportation consultant.

After the derailment, shuttle buses will replace train service on the Green Line Extension — between North Station and Medford/Tufts and Union Square in Somerville, the transit agency said.

The derailment occurred around 5 p.m. when the wheels of the lead car of an eastbound train left the tracks, causing an eastbound subway car near Lechmere to derail at the Red Bridge, the T said Dozens of passengers had to be evacuated and seven people were taken to local hospitals with injuries that were not life-threatening, officials said.

According to initial reports, “there does not appear to be any impact on foreign objects or other equipment,” the NTSB said in a statement.

On Tuesday evening, Ryan Coholan, the transit authority’s chief operating officer, said the investigation was focused on train operations and that problems with the route had been ruled out. During a media briefing at Lechmere train station, he pointed to “human factors”. “I look at the operator, the train operator, how they operated the train,” Coholan said.

Following standard procedure after safety-related incidents, the operator of the train was suspended from duty as the investigation progressed, the T said. Once the investigation is complete, staff will make the necessary repairs to restore service, the authority said.

“We apologize for this unfortunate incident, to the passengers who were aboard this Green Line trolleybus yesterday, and to the passengers who continue to be disrupted by the shutdown,” T CEO Phillip Eng said in a statement Wednesday. “Safety remains our top priority and this incident is unacceptable.”

Norcross-Devin, who lives in Somerville, takes this route daily on the Green Line. Normally, she said, the operator stops so the tracks are switched safely. But that didn’t happen this time. Rather, she said the passengers hopped on the fast-moving train for what felt like a “very long time.” She feared they would fall over the bridge.

“That was terrifying,” she said. “How safe is a chain link fence from a train?”

Passengers were escorted down a stairwell, said Norcross-Devin, who had pain in her ribs and bruises on her leg. She called her husband and ended up walking home on the bike path. Norcross-Devin, who describes herself as a “huge supporter of public transportation,” said she was shocked by the response from officials.

“The fact that they just said, ‘Okay, just another day on the T, nothing to see here.’ Go home.’ It was just amazing to me,” said Norcross-Devin, who has turned to the T and the National Transportation Safety Board for answers.

The AT spokesman referred all questions to the NTSB.

Investigators will collect a range of records and information, including locomotive imagery and event recording devices, track maintenance and inspection records, signal data, witness statements, etc 72-hour crew background to determine “whether there were any issues that may have affected the operator/engineer’s ability to operate the train safely,” the agency said.

The derailment is the latest problem for the transportation agency, which is under pressure to modernize infrastructure and improve the safety and reliability of the country’s oldest public transportation system. The operational funding gap could grow to $700 million next summer.

An ambitious project repaired or replaced tracks and railway sleepers, but also caused delays due to so-called slow zones that increased journey times for passengers. Many of these slow zones have been lifted, but the transit agency remains under pressure to improve overall performance.

There have been repeated derailments on the T-Line, especially on the Green Line, in the past. This year, the agency canceled an $83 million contract with a company in Germany to install crash prevention technology on the track, saying there had been no progress in completing the work. Officials said they plan to find another provider to install the technology.

“It’s very unfortunate because this is proven to save lives,” said Millhouse, who served for more than a decade on the board of Southern California’s Metrolink transit system, which was the first in the country to adopt the technology.

Boston derailments The number of subway cars carrying passengers has been steadily declining since 2019, when it peaked at 21, according to T. In 2023 there were two derailments and three So far this year. Most derailments only cause delays, but in June 2019, at least 10 people were injured when a Green Line train went off the tracks. It was the third derailment this year.

In June 2022, two Green Line trolleybuses collided near the Government Center station and derailed, sending several people to the hospital and disrupting public transportation downtown.

According to T data, derailments occur more frequently on the Green Line than on the Red, Orange and Blue Lines. There were 89 derailments on the Green Line between 2014 and January 2023, according to a Globe analysis of Federal Transit Administration data. The $2.3 billion Green Line extension was so riddled with defects after its full opening in 2022 that the T had to reposition the tracks.

After the NTSB completes its investigation and gives the T its recommendations, it’s important that the transportation agency implements them, “because there should never be a system in place that allows a failure to cause an accident,” Millhouse said.

“The fact that it’s safer than ever is like an airline that had three crashes in a year saying, ‘Oh, we only had two crashes this year,'” he said. “Every incident is serious and every incident should be analyzed and lessons learned to avoid it in the future.”

Shannon Larson can be reached at [email protected]. Follow her @shannonlarson98.