Autopsies by the state medical examiner's office found the women died of accidental "compressional asphyxia," Howard County police spokeswoman Sherry Llewellyn said. The women were not hit by the train, she said.
The bodies of Elizabeth Nass and Rose Mayr, both 19-year-old college students, were found buried under coal after the CSX freight train derailed around midnight Monday. Investigators have said they were sitting on a railroad bridge when the train derailed.
Nass was a student at James Madison University in Virginia. Mayr was a nursing student at the University of Delaware.
Tweets and photos from the women indicated they were drinking on top of the bridge as they enjoyed a summer night together before they headed 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.
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 part of the train jumped the tracks.
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."