Investigators are looking at a broken section of rail to see if it caused two commuter trains to collide in Connecticut.
The trains were headed in opposite directions during Friday’s evening rush hour, when the northbound one derailed and hit the southbound train.
Officials say it isn’t clear if the crash caused the fracture of the rail, or if the rail was broken before the crash.
More than 60 people were injured.
Rail traffic in the Northeast Corridor is shut down until the investigation is complete.