The derailment of a Red Line train in June that disrupted life for thousands of commuters on the region’s busiest subway line was caused by a broken axle, an investigation by the MBTA has determined.
T officials revealed the finding Monday, as they acknowledged that a routine inspection of the Red Line car conducted in May, just one month before the derailment, failed to uncover the problem.
“It is our obligation and our responsibility to make sure this never happens again,” Jeffrey D. Gonneville, the deputy general manager of the Massachusetts Bay Transportation Authority, told reporters.
The agency ordered a new inspection of all subway cars and stricter maintenance rules.
Gonneville said engineers traced the cause of the broken axle to poor electrical connectivity between two metal pieces that allow excess electricity to flow from the motor through the axle and into the wheels and rails.
Those pieces — called the ground brush and ground ring — are made of smooth metal, allowing current to flow without any sparking. But engineers discovered that the ground ring on the derailed train was badly pitted, which caused electricity to zap the axle.
That electrical discharge heated and cooled the steel axle over an approximately six-month period, causing it become brittle.
The fracture — on the front axle of the third car — derailed the Red Line train on the morning of June 11 near the JFK-UMass Station.
No one was injured, but the train smashed into equipment sheds that house electronic signals and switches, forcing the T to run trains at reduced speeds while workers managed train traffic manually.
The slowdown — coming just before the T increased fares on July 1 — inflamed public frustration with the region’s aging system.
Service has improved in recent months but is not expected to return to normal until next month.
Currently, it takes about 35 minutes for the Red Line to travel from Braintree to South Station, down from 55 minutes following the derailment, but still longer than the 30-minute trip before the accident.
To prevent future derailments, Gonneville said, officials are inspecting the ground brushes and ground rings on every subway car in the T fleet and toughening maintenance rules for those two pieces.
The new rules will require subway cars to be taken out of service and immediately repaired if their ground rings have pits of 1/16th of an inch or more, he said.
So far, T workers have checked the ground brushes and ground rings on the two oldest fleets of Red Line cars “and there is no vehicle in the fleet that has anything like that,” Gonneville said.
A careful inspection of the entire Red Line fleet conducted with ultrasonic equipment after the derailment also found no cracks in any axles, Gonneville said.
In the future, the T will conduct ultrasonic inspections on Red Line trains every year, instead every two years, he said.
The T will also require inspections of ground rings when Red Line trains come in for regularly scheduled maintenance every 8,500 miles, or approximately every three months, Gonneville said. Those checks have typically focused more on the ground brushes, he said.
“The good news . . . is they know what it is that caused it, and they have already put in place a whole series of protocols to make sure it won’t happen again,” Governor Charlie Baker said at the State House.
The Red Line train that derailed was among the oldest in the fleet, dating to 1969. The axle itself was installed in 1992, meaning it was still within its useful life, Gonneville said.
The train was inspected in October and December of 2018 and in March and May of 2019. During the December 2018 inspection, T workers noticed that a cover was missing on the assembly that holds the ground brush and ground ring.
Gonneville said the missing cover may have allowed grease, dust, and debris from the tracks to be sucked into the unit, causing pits to form in the ground ring.
But the next two inspections, in March and May, did not uncover any wear and tear on the piece, Gonneville said. Asked why, Gonneville said the inspections typically focus more on checking the ground brushes than the ground rings.
“We know going forward that we have to put a greater level of focus on the ground ring,” he said.
Jimmy O’Brien, president of the Boston Carmen’s Union, said cuts in the T’s operating budget and the elimination of jobs have led to fewer inspections and tests, including ultrasonic testing that could have detected the problems that caused the derailment.
“There are no corners to be cut when it comes to ensuring safety and service for our riders,” O’Brien said in a statement. “A real investment in the MBTA means investing in the personnel and services required to ensure safety and satisfaction for passengers.”
T officials rejected the union’s claim.
“At no time has the MBTA made any budget decisions that would adversely impact vehicle inspection protocols or practices,” said Joe Pesaturo, a T spokesman. “Our focus remains on rider and worker safety; we will not engage with opportunistic and baseless allegations.”