The 50- to 60-year-old steel cable probably “weakened until the point where it came down” after decades in a damp, subterranean tunnel, said Phillip Eng, the MBTA general manager.

It took firefighters approx. 1 hr to evacuate 500 people from a ⁦ @MBTA ⁩ Blue Line train that was disabled under the harbor between Maverick & Aquarium station. They used the Nolan cart to transport anyone needing assistance using 3 exits , 2 in Boston & 1 in East Boston pic.twitter.com/8fYoSYLJHJ

More than 450 MBTA passengers were evacuated from a Blue Line train in the tunnel under the Boston Harbor due to a signal issue Tuesday afternoon.