ELLICOTT CITY, Md. (AP) — Two young women were killed when a freight train derailed and toppled so much coal on them they couldn't breathe, authorities said Thursday.

The bodies of Elizabeth Nass and Rose Mayr were found buried under coal but still seated on edge of a railroad bridge in Ellicott City, police have said. They were not hit by the train, Howard County police spokeswoman Sherry Llewellyn said.

Llewellyn said an autopsy found the women died from accidental asphyxia.

Both were 19-year-old college students. Nass attended James Madison University in Virginia and Mayr was a nursing student at the University of Delaware.

Tweets and photos from the women indicated they were drinking on the bridge as they enjoyed a summer night together before heading back to school. "Drinking on top of the Ellicott City sign," read one tweet. "Looking down on old ec," read another.

Their funerals are set for Friday and Saturday.

CSX said Thursday it resumed train operations along the railroad line at 4 p.m.

Howard County spokeswoman Alexandra Bresani said the street underneath the railroad bridge would remain closed until the on-site investigation and removal of the coal was finished.

"We're making good progress but it's quite a bit," CSX spokesman Gary Sease said, noting most of the 21 derailed cars dumped their entire 110-ton load in the accident.

The bridge the women were on is easily accessible from the picturesque downtown of Ellicott City, which is about 15 miles west of Baltimore, and generations of young people have played and partied along the tracks.

The National Transportation Safety Board is investigating the cause of the wreck. Federal investigators have said the train was going the authorized speed of 25 miles per hour with an engineer-in-training at the controls before the wreck.

Investigators have also said the train's emergency brakes were applied automatically — not by the three-man crew — but they don't know why 21 cars of the 80 cars derailed.

National Transportation Safety Board investigator Jim Southworth has said the train's two locomotives did not derail and that the crew reported they "felt nothing, and they saw nothing before emergency braking occurred on their train."