NATIONAL BRIEFING / WASHINGTON, D.C.

A federal safety panel said the trolley operator who died after her train rammed another trolley in Boston last year had ignored a red stop signal, probably 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 Terrese Edmonds’ urine. The safety panel said Edmonds had a high body mass index, which is closely correlated with a higher risk for sleep apnea. Sleep apnea can cause problems sleeping at night and drowsiness during the day.