A fractured axle is what caused a Red Line train to derail as it entered JFK/UMass station on June 11, an MBTA investigation revealed Monday showed.
Engineers found that poor electrical connectivity between a stationary ground brush and a rotating ground ring — two components on the axle — was the driving catalyst in wearing down the axle, making it brittle enough to have cracked as it did on car 1602, according to a report presented to the MBTA’s Fiscal and Management Control Board.
“We obviously very much regret that this incident occurred. We were committed to finding out what the root cause was and we have done that,” MBTA General Manager Steve Poftak said at the board’s meeting Monday. “It has taken some time, but I think that there has been a lot of good work here to figure that out.”
Schedule changes and delays rippled through the Red Line for months. The agency said in August normal service is anticipated to be fully restored next month.
According to the MBTA, the ground ring and the ground brush are key conduits in carrying excess electricity to the wheels and the rails.
While a ground ring in good condition has a smooth surface, the ground ring involved in the derailment was pitted, which caused electrical sparks that in turn hardened the steel axle, thus weakening it over time, officials said.
“As the material gets harder, (the axle) begins to get more brittle, and as things progress, that brittle material, the brittle properties of the axel can begin to form a small crack,” MBTA Deputy General Manager Jeff Gonneville said.
The probe found that the age of the axle — built in 1992 — was not at fault, nor was there any material flaw, defect, or “failure of bearings that allow the axle to rotate,” according to Gonneville’s presentation.
Once the cracked axle was found, the MBTA began ultrasonic inspections of axles on trains across the system, Gonneville said.
“Even the smallest of cracks that were beginning would have … been determined and found through this ultrasonic inspection,” he said. “The entire Red Line fleet has gone through this ultrasonic inspection.”
As a precaution, the agency is also inspecting axles on the Orange, Blue, and Green line trains, he said. The process is expected to wrap up in three weeks.
Ultrasonic inspections are currently conducted every two years, according to Gonneville, who said the car that derailed in June was initially slated to be inspected this summer.
As a result of the derailment, the MBTA will instead conduct ultrasonic inspections annually, Gonneville said.
The MBTA will also tweak its 8,500-mile preventative maintenance inspections, which occur approximately every three months, officials said.
Inspectors will be required to remove the components that house the ground ring and manually review the piece, according to Gonneville.
Previously, the inspection focused more on the condition of the ground brush and therefore the issue was not seen during the axle’s review prior to the derailment in May, he said.