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.

The MBTA's general manager said that a decades-old cable led to the shutdown of the Blue Line on Tuesday, which prompted the evacuation of 500 people under Boston Harbor.

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.