MBTA officials said Monday that a June derailment on the Red Line was caused by a fractured axle, three months after the incident caused major delays.
Officials said poor electrical connectivity between a ground brush and ground ring caused stray electrical arcing on the surface of the axle. Over time, this weakened the axle and led to the failure.
The revalation was the result of a "comprehensive assessment" of the incident that included analysis conducted by an independent engineering firm, officials said.
Officials from the MBTA’s Fiscal and Management Control Board met with MassDOT’s Board of Directors for a joint session Monday, in which officials discussed the results of the assessment.
MBTA Dep. General Manager Jeff Gonneville said going forward, it was on the MBTA to make sure technicians know the right procedures.
"That is on us. That is on the MBTA to ensure we are giving our technicians the right procedures they should be following," Gonneville said. "In full transparency, we, the authority, has to ensure that our technicians are doing a more precise inspection of the ground ring.
The MBTA has completed an "ultrasonic" inspection of all Red Line vehicle axles in an attempt to identify any other fractures or other potential issues, officials said.
Orange, Blue and Green Line vehicles are also being inspected, as are ground rings on MBTA trains.
The Red Line train derailed June 11, hitting bungalows that housed signal systems and causing significant damage.
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The incident, prompted severe delays and frustration among passengers.
Transit officials said crews will likely be done repairing the signals by October.
Jimmy O'Brien, President of the Boston Carmen's Union, released a statement Monday in response to the Red Line derailment.
"The MBTA claims a commitment to investing in the system while simultaneously boasting about cost savings in the operating budget. Cuts to the operating budget and the elimination of jobs resulted in cuts to inspections and testing – including the ultrasonic testing that could have detected the problem that resulted in the Red Line derailment. There are no corners to be cut when it comes to ensuring safety and service for our riders. A real investment in the MBTA means investing in the personnel and services required to ensure safety and satisfaction for passengers," O'Brien said.