MBTA blames broken wheel for derailment

One of the eight steel wheels on the fourth car of the six-car train had cracked and a doughnut-shaped piece of steel around the wheel’s perimeter had fallen off, said Anna M. Barry, the MBTA’s director of subway operations.

Sixty-five people were evacuated after the derailment, which happened at about 4 p.m. Nobody was injured.

The MBTA inspected the wheels on other cars of the same model, the Bombardier 1800. But no signs of impending trouble have been found, Barry said.

It’s quite likely, she said, the wheel cracked from being overheated when the train was braking, but the problem was very unusual. The crack puzzled subway engineering veterans with decades of experience who examined the scene Tuesday.

“No one that was there that night had ever seen anything like it,’’ she said.

Pieces of the failed wheel will be sent to a metallurgical lab, she said, and the MBTA has accelerated its replacement programs for the wheels. If faulty manufacturing is to blame, she said, the MBTA will work with the vendor to “hold them accountable for any defects.’’

She said the MBTA had “taken every step’’ to make sure the problem wouldn’t happen again.

The MBTA has had several problems in recent weeks, including a Green Line train derailment on Dec. 13 near Copley Square and problems with a 40-foot wall Monday on the Red Line at the Porter Square station. Officials said the Green Line incident was caused by human error and the wall problem was due to the system’s aging infrastructure.

The Alewife derailment is “completely unrelated to anything else that has happened,’’ said Barry.

MBTA spokesman Joe Pesaturo said there was one common thread in the two Red Line incidents: “The T acted swiftly in both cases to make sure the riding public was safe.’’