Obesity , evidence that operator was wearing sleep aid led to investigation . Transportation board criticized for failing to screen for sleep aids . The safety board says positive train control issues were involved in two other light rail incidents this year , a May 8 wreck on the same MBTA Green Line and a June 22 wreck which killed nine on Washington , D.C. 's Metro system . It also criticized lack of systems to prevent trains from using same stretch of track .
Obesity , evidence that operator was wearing sleep aid led to investigation . Transportation board criticized for failing to screen for sleep aids . It also criticized lack of systems to prevent trains from using same stretch of track .