So far, investigators have determined the emergency brakes were applied automatically -- not by the three-man crew -- on Monday around midnight, but they don't know why the train jumped the tracks. It could be weeks before they know anything definitive.
Tweets and photos from the two 19-year-old college students chronicled some of their final moments together as they enjoyed a summer night together before they were to headed back to school.
"Drinking on top of the Ellicott City sign," read one tweet. "Looking down on old ec," read another.
Accompanying photos showed their view from the bridge and their bare feet, one with painted blue toenails, dangling over the edge. "Levitating," read the tweet.
The women were sitting on the edge of the bridge with their backs to the tracks as the train passed a few feet behind them, Howard County police said, and their bodies were found buried under coal dumped from the train cars. Authorities said they needed to do autopsies before their cause of death could be determined.
The victims were identified as Elizabeth Conway Nass, a student at James Madison University in central Virginia and Rose Louese Mayr, a nursing student at the University of Delaware.
The railroad is easily accessed 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 railroad was completed in 1830 and crosses over Main Street in the city's historic district, following the route of the nation's first commercial railroad, according to the Baltimore & Ohio Railroad Museum.
"We grew up running on those tracks," said Ellicott City native Bridgette Hammond, 25. "It's actually really beautiful up there."
Nass and Mayr were graduates of Mt. Hebron High School in Ellicott City, where they were on the dance team, and planned to finish college in 2014, according to friends and their Facebook pages.
One of Nass' sorority sisters, Donya Mossadeghi, called her "a joy to talk to" and someone who "would never say a bad thing about anybody." Nass made the dean's list in the fall of 2010 and 2011, according to a university spokesman, and another friend said she was studying special education.
Tori Mace, of Ellicott City, knew Mayr through mutual friends. "She was really fun, really friendly," Mace said.
A person who answered the telephone at Nass' home declined to comment, as did a family member who answered at a number listed for the Mayr family.
The pictures and tweets from Mayr were no longer publicly available, but friends confirmed they were hers and police said they were aware of the posts and looking into them.
Benjamin Noppenberger was getting ready for bed when he and his wife heard what sounded like gunshots. They waited about 10 minutes before going outside.
"We could see all the cars that fell over. I just saw catastrophe," he said.
Jim Southworth, investigator in charge for the National Transportation Safety Board, declined to speculate on a possible cause. He said the brakes were applied automatically when an air line used to pressurize the braking system was disconnected. He did not say what role, if any, the brakes may have played in the derailment.
"This will be a very wide-ranged investigation," he said, adding later that investigators "will look into the maintenance of the track, the maintenance of the equipment, the maintenance of the locomotive -- everything you can think of."
The crew -- an engineer, a conductor and an engineer trainee -- didn't see or feel anything unusual before the crash, Southworth said. They were not injured.
The train was equipped with video-recording devices that investigators will review to help them determine what happened. It was going about 25 mph but Southworth would not say whether that was an appropriate speed limit for the area.
CSX spokesman Bob Sullivan said the train was traveling from Grafton, W.Va., to Baltimore. It had two locomotives and weighed 9,000 tons, he said. The first 21 cars of the 80-car train derailed.
Environmental officials responded because about 100 pounds of coal spilled into a tributary of the Patapsco River, a major Maryland waterway that parallels the tracks. Maryland Department of the Environment spokesman Jay Apperson said much more coal lay along the edge of the tributary, raising concerns it could boost the acidity of the water or otherwise threaten aquatic life.
The derailment also damaged some of Verizon's equipment, disrupting land-line telecommunications services to clients, including some government servers.
Gresko reported from Washington. Associated Press writers Matthew Barakat in Ellicott City, David Dishneau in Hagerstown, Md., and Karen Mahabir in Washington contributed to this report.