BOSTON (CBS) – The MBTA says a fractured axle is what caused a train to derail on the Red Line earlier this summer.

The axle was weakened over time due to “poor electrical connectivity between the ground brush and ground ring [which] caused stray electrical arcing on the surface of the axle,” said the MBTA.

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Age, flawed material or detects, and failure of the bearings were ruled out as causes of the axle’s failure.


The train was inspected a month before the derailment, but the ground ring — where the problem lied — was not a part of the procedure.

“That is on us, that is on the MBTA to ensure that we are giving our technicians the right procedures that they should be following to do a very precise inspection of the ground rings,” said MBTA Deputy General Manager Jeffrey Gonneville.

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Train Axle (Photo Credit: MBTA)

The train derailed in June, causing damage to the signal system.

Crews have been working ever since to make repairs and get the Red Line back on schedule. Restorations of signal control between Broadway to JFK and between Fields Corner to JFK were completed on July 31 and Sept. 11, respectively. Work to restore signal control between North Quincy to JFK is expected to finish up in October.

An MBTA Red Line train derailed just outside the JFK-UMass station June 11. (Photo credit: Boston Fire Dept. )

Going forward, the MBTA said they will conduct annual engineering axle inspections and preventative maintenance inspections at 8,500 miles, and will now include detailed ground ring inspections. All rail car axle ultra-sonic inspections will take place once a year as opposed to once every two years.

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“We will do everything we need to ensure the safety of our fleet. That is why we did this robust inspection to determine the cause of this failure,” said Gonneville.