What Happened
What Happened
On October 1, 2024, about 4:46 p.m., eastbound Massachusetts Bay Transportation Authority (MBTA) Green Line train 3874-3718 derailed its leading railcar at MBTA’s Red Bridge interlocking in Somerville, Massachusetts. Train 3874-3718 was a light rail vehicle composed of two railcars with about 50 passengers and 2 crew members on board. Less than a minute before the derailment, the train departed Lechmere Station in Cambridge, Massachusetts, and headed east along the MBTA Green Line. The train entered a 10 mph zone while traveling about 36 mph, passed through a double red signal (which requires a stop) west of the Red Bridge interlocking, and reached a track switch that was still in the process of moving to direct the train into the diverging track as required by its route. The train derailed while passing over this switch. Seven passengers were transported to a local hospital with minor injuries.
What We Found
What We Found
The NTSB determines that the probable cause of the Massachusetts Bay Transportation Authority train derailment was the operator failing for unknown reasons to obey a stop signal and proceeding through an interlocking while the track switch was still in motion. Contributing to the accident was the train’s excessive speed, which increased the likelihood of the train reaching the interlocking while the switch was still in motion.
What We Recommended
In response to this accident, MBTA published a safety alert for all employees on October 24, 2024, addressing speed requirements and the hazards of failing to obey speed limits.
MBTA changed the speed limits on the eastbound approach to the Red Bridge interlocking, reducing the maximum speed in the area from 30 mph to 25 mph. This is intended to decrease how much an operator accelerates and decelerates between Lechmere Station and Red Bridge and to provide more time to see and react to a signal requiring a stop.
MBTA also concluded that its operational testing (audit) program was inadequate to identify noncompliance with speed limits and signal indications. MBTA has since revised its program to include more accurate GPS-based speed monitoring, retargetedits speed audits to focus on locations where speeding is more likely, and installed inward- and outward-facing cameras on all equipment. MBTA has also begun installation of systems to alert operators when they exceed speed restrictions. Further, MBTA has described plans to install an ATC system by June 2028.
This investigation and MBTA’s response highlight the importance of deploying and maintaining effective systems to manage human error. These systems include robust oversight technologies and practices to identify and correct unsafe behaviors, and safety technologies like ATC to reduce or eliminate the consequences of mistakes.