A federal safety panel said Tuesday that the trolley operator who died after her train rammed another trolley in a Boston crash last year had ignored a red stop signal, likely because she suffered from an undiagnosed sleep disorder that caused her to briefly fall asleep.

The finding came in the National Transportation Safety Board's final report on the May 2008 collision in suburban Newton, Mass. that also injured seven passengers on the Massachusetts Bay Transportation Authority's Green Line.

Testing showed the presence of Doxylamine — which is used in over-the-counter sleeping aids — in operator Terrest Edmonds' urine.

The panel also said the fatal crash could have been prevented if the MBTA used an automated train control system.

Acting NTSB Chairman Mark V. Rosenker said if such an automatic braking system had been implemented, "I don't believe we'd be here discussing this today."

Rosenker noted that other MBTA lines had such systems.

"If technology exists and it exists on the other lines, why would the Green Line not have everything possible that is going to prevent the accidents from happening?" Rosenker said.

Edmonds, 24, should have halted her trolley for one minute and continued slowly at 10 mph because the signal was red, indicating a train ahead, investigators found.

Instead, her trolley plowed into another train. Investigators estimated the striking trolley was moving at 38 mph at impact.

The trolley struck from behind had stopped at a second red signal that had been malfunctioning intermittently, investigators said.

Investigators found broken electrical connections between track sections, interrupting the signal that would have made the second signal light go on and off correctly.

Operators were not required to report faulty signals by the MBTA, a factor that likely contributed to the accident, investigators said.

"This could lead to a great safety risk," investigator Jeff Leaman told the panel.

The crash damage was estimated at $8.6 million by investigators. An estimated 185 to 200 passengers were on the two trains at the time of the crash.

Investigators also told the panel that testing showed that no illegal drugs or alcohol were detected in the operator's system when she died.