After the T line derailed, buses are running indefinitely on both branches of the GLX

Shuttle buses will indefinitely replace trains on the MBTA’s Green Line extension after a derailment near Lechmere Station injured several people Tuesday night.

MBTA officials previously said about 50 passengers self-evacuated from the train, which remained upright after the guide trolley’s wheels came off the tracks. Authorities said six people were injured and seven passengers were taken to local hospitals.

Buses began replacing trains on the Union Square and Medford/Tufts branches of the Green Line on Tuesday evening, and on Wednesday the MBTA announced that buses will continue to run between North Station and both GLX terminus stations for now.

“At this time, it is unclear when service will resume,” T said in a news release, adding that shuttle buses will be free and available to all passengers.

The agency said it was investigating the derailment with state and federal regulators, working with the National Transportation Safety Board, the Federal Transit Administration and the Massachusetts Department of Public Utilities to determine the cause.

MBTA Chief Operating Officer Ryan Coholan said Tuesday that a preliminary investigation showed the derailment was not the result of track problems, despite concerns last year about the tracks being too narrow on the GLX. The T Line said it was considering “human factors,” including the actions of the train operator.

“We apologize for this unfortunate incident to the passengers who were aboard the Green Line trolley yesterday, and to the passengers who continue to be impacted by the outage,” MBTA CEO Phil Eng said in a statement. “Safety continues to be our top priority and this incident is unacceptable.”

Once the derailment investigation is complete, MBTA staff will make necessary repairs to restore service, according to Wednesday’s news release.

“We are working with our partners at the federal and state levels to understand what happened and to ensure this does not happen again,” said Eng. “We know this unplanned service change is challenging and we thank our passengers for their patience at this time.”