The National Transportation Safety Board (NTSB) determined that the two-train collision on the Massachusetts Bay Transportation Authority (MBTA) Green Line in 2008 “occurred as a result of the trolley operator’s failure to obey a signal indication likely because she became disengaged from her environment consistent with experiencing an episode of micro-sleep.”

Additionally, the NTSB [removed]cited[/removed] the lack of a positive train control system, which would have intervened to stop the train and prevented the collision, as a contributing factor.

The NTSB wrote, “At 5:51 PM EDT, on May 28, 2008, an MBTA Green Line train, traveling westbound at about 38 mph, struck the rear of another westbound Green Line train, which had stopped for a red signal. The operator of the striking train was killed; three other crewmembers sustained minor injuries. Of the 185 to 200 passengers who were on the two trains at the time of the collision, four sustained minor injuries and one was seriously injured. Total damages were estimated to be about $8.6 million.”

According to postaccident toxicological testing, the operator of the striking train had recently taken the drug doxylamine. The presence of the drug found in some sleep aids suggested that she had trouble sleeping during at least one of the nights leading up to the accident. The operator also had a high body mass index, raising eyebrows regarding the likelihood of sleep apnea and increasing the likelihood of a fatigued condition during waking hours.

"Again, we’ve seen a situation where a positive train control (PTC) system could have prevented a tragic accident," said Mark V. Rosenker, acting chairman of the NTSB in an announcement. "We know that because operators or equipment sometimes fail, redundancies like PTC systems can be the difference between a fatal accident and an incident report. And this is why we feel so strongly that transit systems like MBTA’s should have that crucial extra layer of safety that a PTC system provides."

Following this report, the NTSB recommended to the Federal Transit Administration (FTA) that the agency facilitate the development and implementation of positive train control systems for rail transit systems nationwide. The NTSB also recommended the development and dissemination of guidance for operators, transit authorities, and physicians regarding the identification and treatment of individuals at high risk for obstructive sleep apnea and other sleep disorders.

After an earlier transit system accident in Maryland that cited human fatigue, the NTSB recommended that the MBTA establish a fatigue awareness program to address potential sleep disorders among its train operators. MBTA indicated that it had established and implemented such a program; however, the NTSB determined that the efforts were inadequate and is now reiterating this safety recommendation.

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