After a Green Line derailment near Lechmere station in Cambridge sent seven people to the hospital, the MBTA moved to replace the line with shuttles as federal and state investigations examine the cause of the incident.

“We apologize for this unfortunate incident, to the riders who were aboard this Green Line trolley (Tuesday), and to riders who continue to be disrupted by the shutdown,” said MBTA General Manager and CEO Phillip Eng, calling the derailment “unacceptable.”

He added: “We’re working alongside our federal and state partners to understand what happened to make sure this doesn’t happen again.”

The Green Line service will be replaced by free shuttle buses between North Station and Union Square and between North Station and Medford/Tufts for an “unclear” period of time, the MBTA said.

The derailment happened at about 5 p.m. Tuesday when the wheels of the lead trolley of a Green Line train came off the tracks near Lechmere eastbound at the Red Bridge, the MBTA reported. One car derailed but “remained upright,” the T said.

Preliminary findings ruled out problems with the tracks, Ryan Coholan, MBTA Chief Operating Officer, told reporters Tuesday evening.

“Right now we’re focused on the human factors involved,” Coholan said.

There were about 50 passengers in the derailed train, Coholan said, and seven were taken to the local hospital with “minor injuries.” Others were evacuated through an emergency exit.

The National Transit Safety Board is actively investigating the incident, the organization said, and had a team on site Wednesday.

“Once on site, the investigator will begin the process of documenting the scene and examining the train cars and equipment involved,” the NTSB said Wednesday. “NTSB investigations involve three primary areas: the human, machine and the operating environment.”

The board urged witnesses to reach out with any surveillance video or other information that could be relevant to the investigation at witness@ntsb.gov. A preliminary report will be available within 30 days, the NTSB said, while the final report determining probable cause of the accident and contributing factors will be available in 12 to 24 months.

“Once the investigation is complete and the area is returned to the T, infrastructure and maintenance personnel will make all repairs necessary to restore service,” the MBTA stated.

T officials did not give further information on when service may resume, saying updates will be posted as the investigations progress.

At North Station on Wednesday afternoon, T officials announced the service change over intercom announcements, with T workers posted at the Green Line platform inside and shuttle stop outside, and signs around the building.

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“There’s always something, isn’t there,” said Somerville resident Ken Adams, heading home from work via the Green Line shuttle Wednesday.

Adams said he hadn’t heard about the derailment before work, and it took time to figure out the shuttle service first thing in the morning. He said he hadn’t seen signs at the station.

Other riders echoed the confusion and frustration with the disruption Wednesday.

“It’s just a pain,” said Sarah Moen, walking to the shuttle stop in North Station. “I work all day, and it’d be nice to be able to get home without jumping through hoops.”

Eng in a statement acknowledged the “challenge” and thanked riders “for their patience at this time.”

More information will be released as it becomes available, the MBTA said, directing riders to check updates through mbta.com/alerts and on X @MBTA.

An inspector takes a closer look at the derailment. (Nancy Lane/Boston Herald)