Two teenagers sat atop the CSX railroad bridge running through the heart of Ellicott City around midnight Monday, enjoying the view and each other's company before heading back to college.

"Drinking on top of the Ellicott City sign," Elizabeth Nass wrote on Twitter.

Rose Mayr tweeted a picture of their two pairs of feet dangling from the edge of the bridge overlooking historic Main Street -- "Levitating," she wrote.

Only minutes later, the two 19-year-old women were crushed underneath several tons of coal spilling out of a CSX train, as the first 21 cars of a 80-car, 9,000-ton train headed to Baltimore derailed, according to Howard County police.

The two Ellicott City residents were found in the coal, seated along the edge of the bridge over Main Street, where they had been facing east with their backs to the train as it rolled just several feet behind them, police said.

Investigators were still trying to figure out what caused the derailment. Witnesses heard squealing brakes and a thunderous crash around midnight Monday.

An automatic emergency break was deployed, according to Jim Southworth, the lead investigator for the National Transportation Safety Board, but it's not clear if that caused or was caused by the derailment. Some of the railcars fell off the bridge and onto the cars below.

The women were both about to start their junior year of college. Mayr was a nursing major at the University of Delaware, according to a spokeswoman.

James Madison University spokesman Don Egle said Nass was in the honors program at the Harrisonburg, Va., school and was majoring in interdisciplinary liberal studies.

A woman who answered the phone at the Mayr's residence said the family wasn't speaking to reporters Tuesday. A call to a number listed for the Nass residence went unanswered.

Ellicott City resident Jean Anderson said she felt mortified for the girls' parents -- when she heard the news, Anderson immediately thought of her own son, who was driving home shortly after midnight when his route toward Main Street was cut short by the wreckage.

"Kids like to have a few drinks, maybe a few other things, and go up on the tracks," she said. "It's dangerous, but it's popular."

Ellysia Mohammed, 17, looked out at the bridge Tuesday afternoon with her mother. She commonly drives to Ellicott City to explore the tracks and surrounding area with friends, she said.

"She brought me because she doesn't want me to hang out here," said Ellysia, who lives in nearby Columbia.

NTSB investigators declined to say what might have caused the derailment. Crews will be at the site of the incident again Wednesday -- once the on-site investigation is complete, the NTSB will release a preliminary report, according to spokesman Eric Weiss.

A full investigation could take 12 to 18 months.

Three crew members aboard the train were uninjured.