They were seemingly ordinary tweets from two friends hanging out on a railroad bridge in their hometown, enjoying one last summer night together before heading back to college.

"Drinking on top of the Ellicott City sign," read one. "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 another tweet.

Minutes after the messages were sent, a CSX freight train loaded with coal barrelled down the tracks and derailed, killing the 19-year-old women and toppling railcars and coal on the streets below.

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

It wasn't clear whether the women's presence on the tracks had anything to do with the derailment.

They were sitting on the edge of the bridge as the train passed a few feet behind them, Howard County police said, and their bodies were found buried under coal. Authorities said they needed to do autopsies before their cause of death could be determined.

Killed were Elizabeth Conway Nass, a student at James Madison University in Virginia and Rose Louese Mayr, a nursing student at the University of Delaware.

The railroad is easily accessible from the picturesque downtown of Ellicott City, and generations of young people have played and partied along the tracks.

"We grew up running on those tracks," said Ellicott City native Bridgette Hammond, 25. "It's actually really beautiful up there."

The pictures and tweets from Mayr were no longer publicly available Tuesday afternoon (local time), but friends confirmed they were hers and police said they were aware of the posts and looking into them.

Jill Farrell, who lives across the street from the tracks, said she heard what sounded liked squealing brakes and then a crash, followed by silence.

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 NTSB, declined to speculate on a possible cause.

He said the brakes were applied automatically when an airline 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," Southworth said, adding that officials 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 of three - 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 miles per hour but Southworth would not say whether that was an appropriate speed limit for the area.

Train crash halts Guantanamo hearings

The wreck disrupted internet connections to the Guantanamo Bay US Naval base on Tuesday and caused a one-day delay in pre-trial hearings for five prisoners accused of launching the September 11 attacks.

Hearings had been scheduled to begin on Wednesday (local time) for the alleged mastermind of the hijacked plane attacks, Khalid Sheikh Mohammed, and four others facing death penalty charges of mass murder, terrorism and conspiring with al Qaeda.

The train derailment knocked out fibre optic lines that are part of the wire-and-satellite network providing communications to the remote US Naval base in eastern Cuba, said Captain Robert Durand, a spokesman for the Guantanamo detention operation.

That left defence lawyers and prosecutors at the base temporarily unable to access email and electronic legal files.

Defence lawyers said the network breakdown had drastically interrupted preparations for the hearings, and filed an emergency request for a delay.

The judge, Army Colonel James Pohl, postponed the start of the hearings until Thursday.

The session was scheduled to last six days to address secrecy rules governing the trials, among other issues.

The United States maintains an economic embargo on Cuba that is intended to put pressure on the island's communist government and there are no communications links between the US base and the rest of Cuba.

Satellites relay phone and Internet signals to and from the base, linking it with communications networks in Maryland and Maine.

The Maryland link was expected to be fully restored by Tuesday evening, Durand said. The hearings could face additional disruption from Tropical Storm Isaac, which formed in the Atlantic Ocean east of the Lesser Antilles.

It was forecast to strengthen as it moved into the Caribbean Sea by Thursday, and could threaten Guantanamo as a hurricane on Saturday, the US. National Hurricane Centre said.

-With AP

- Reuters