MBTA management today released the results of their investigation of the causes of the Red Line derailment in June.
They have concluded that the car derailed because its axle failed. The axle failed because it had been damaged by arcing electrical currents. Normally, direct current runs from the third rail through the motor back to ground through the axle and the wheels. The electrical arcing was caused by deterioration of a ring that normally conveys current from the motor to the rotating axle. Overtime, the arcing hardened the steel of the axle and made it brittle.
The attached presentation explains the issue in more depth and also shows that service is approaching very close to its pre-derailment levels. Delays still persist between North Quincy and JFK/UMass. In this stretch signal replacement work is still in progress with completion hoped for next month.
In the presentation, the Deputy General Manager explains that the impending axle failure would have been detected by the regular ultrasonic inspection of axles which occurs every two years. To reduce risk, the T has done a reinspection of all the axles in this type of car and will do so every one year. Additionally, during their 8500 mile inspection (roughly quarterly), engineers will conduct a full inspection to the current ring and take out of service any rings that show material deterioration.
To see some of the questions and answers from the MBTA Board Meeting, you can view the proceedings starting at approximately 2.5 hours in the livestream at this link. Questions will continue as to how this particular ring got so badly damaged. A State House News reported that the Deputy General Manager said:
How exactly the ring came to be so corroded remains unclear . . . A December 2018 preventative maintenance inspection, one conducted roughly every three months, noted that the assembly itself was missing the cover that keeps it sealed. . . . . there are no other immediate risks to the system from similar causes. Crews performed ultrasonic inspections on the entire Red Line fleet after the derailment and pulled seven axles flagged for potential issues from service, and two ground rings that demonstrated even the start of damage were also replaced.
For previous updates, please see this link.