Train service resumed Thursday morning on the Green Line Extension, a day and a half after a derailment injured several passengers and triggered a federal investigation.

Green Line service resumed Thursday between North Station and Medford/Tufts following the closure after a derailment Tuesday. “We sincerely apologize for this incident and thank the public for their patience while we worked to resume service,

The T said it is working with state and federal agencies to determine what caused a train to derail near Lechmere Tuesday.