WASHINGTON — The National Transportation Safety Board determined that the two-train collision on the Massachusetts Bay Transportation Authority (MBTA) Green Line last year 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.
The lack of a positive train control system, which would have intervened to stop the train and prevented the collision, was cited as a contributing factor.
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.
Post-accident toxicological testing indicated that the operator of the striking train had recently taken the drug doxylamine, commonly found in sleep aids, suggesting that she had trouble sleeping during at least one of the nights leading up to the accident. In addition, the operator had a high body mass index (BMI), which is closely correlated with a higher risk for sleep apnea, increasing the likelihood of a fatigued condition during waking hours.
NTSB Acting Chairman Mark V. Rosenker highlighted the role a positive train control system could have played in preventing the accident.
“Again, we’ve seen a situation where a positive train control system could have prevented a tragic accident,” he said. “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.”
Because the Safety Board concluded that the accident could have been prevented had the MBTA been equipped with a positive train control system, the NTSB recommended to the Federal Transit Administration that the agency facilitate the development and implementation of positive train control systems for rail transit systems nationwide.
As a result of its investigation of a transit system accident in Maryland in 2000, the Safety Board recommended in 2001 that MBTA establish a fatigue awareness program to addresses potential sleep disorders among its train operators.
While the MBTA indicated that they had established and implemented such a program, the Safety Board determined that their efforts were inadequate and reiterated that safety recommendation.
In addition, the NTSB recommended to the Federal Transit Administration (FTA) that they develop and disseminate 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.
The Safety Board recommended to MBTA that train operators be required to notify fellow crewmembers when a restrictive signal is encountered, and the intended means of complying with the restriction.
And because MBTA operating rules do not require train operators to report signals displaying red when the track is visibly clear, the NTSB has concluded that this could allow possible problems in the signal system to remain undetected and unrepaired, which could increase safety risks. Thus, the Safety Board recommended to MBTA that train operators immediately report all red signals they encounter when the track governed by that signal can be seen to be clear of other trains.