BOSTON (SHNS) - Train service on the entire Green Line Extension will be shut down "until further notice" while MBTA crews and federal investigators determine the cause of -- and damage from -- a Tuesday afternoon derailment. Shuttle buses will run between North Station and Union Square or Medford/Tufts, both endpoints on the Green Line Extension's two branches, until the end of a federal investigation and subsequent repairs, the MBTA announced Wednesday. MBTA General Manager Phil Eng apologized for the disruption and called it "unacceptable." Officials said the first car of a Green Line train derailed just after 4:45 p.m. Tuesday outside of Lechmere Station, blocking tracks to both branches. Six riders were hospitalized with "non-life-threatening injuries," and about 50 passengers total evacuated to an "emergency egress path," the T said on social media. The MBTA will investigate the derailment alongside the National Transportation Safety Board and the Federal Transit Administration, the latter of which in 2022 issued a stern warning about safety at the T and ordered a raft of changes. Many of those improvements have since been made, while some are still underway. "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," T officials said Wednesday. It was not immediately clear how much damage the derailment caused. An NTSB spokesperson said there did "not appear to be any impact to foreign objects or other equipment," but stressed that the incident is still being examined. "A team of NTSB investigators are expected to be on scene today. Once on site, the investigator[s] will begin the process of documenting the scene and examining the train cars and equipment involved," the spokesperson said. "NTSB investigations involve three primary areas: the human, machine and the operating environment." NTSB investigators will examine records including image and event recorders from the train, track maintenance and inspection records, signal data, braking system records, weather forecasts, witness statements, and a 72-hour background of the crew. The agency's preliminary report will become available within 30 days, and a final report will take one to two years. The indefinite closure is the latest disruption to the Green Line Extension, which opened to riders in two stages in 2022. Last fall, MBTA officials said they discovered that most of the tracks were built too narrowly, requiring maintenance work to widen them to the original project construction standards. Those repairs wrapped up in the winter. "We apologize for this unfortunate incident, to the riders who were aboard this Green Line trolley yesterday, and to riders who continue to be disrupted by the shut down," Eng said in a statement Wednesday. "Safety continues to be our top priority and this incident is unacceptable. We're working alongside our federal and state partners to understand what happened to make sure this doesn't happen again. We know this unplanned service change is a challenge, and we thank our riders for their patience at this time."