Jervis Bryant, a Prince George's County teacher, told the Baltimore Sun that he heard the collision from a house 2 1/2 blocks away and got to the scene within five minutes.
"We saw the folks banging on the windows trying to get out," he said, referring to the second train.
Rescuers pried the door open, he said, and people streamed out. "They just bum-rushed it."
Passenger Jodie Wickett, a nurse, told CNN she was seated on one train, sending text messages on her phone, when she felt the impact. She said she sent a message to someone that it felt as if the train had hit a bump.
"From that point on, it happened so fast. I flew out of the seat and hit my head." Wickett said she stayed at the scene and tried to help. She said, "People are just in very bad shape."
"The people that were hurt, the ones that could speak, were calling back as we called out to them," she said. "Lots of people were upset and crying, but there were no screams."
The National Transportation Safety Board is investigating and has assigned a railroad investigator and two specialists from its office of transportation disaster assistance.
Investigators will probably focus on a failure of Metro's computerized signal system, designed to prevent trains from coming close enough to collide, as well as operator error, according to former Metro officials.
The system relies on electronic relays -- each about the size of a hardcover book -- aboard trains and buried beside the tracks along each line. When a train gets too close to another train, the system is designed to automatically stop the approaching train. It should work whether trains are being operated manually or by computer.
But even if the signal system failed to stop the train, the operator should have intervened and applied emergency brakes, safety experts familiar with Metro's operations told the Post.
The position of the second train after the crash -- the fact that its first car came to rest atop the other train -- indicates that the second train was traveling at high speed. In the section of track where the accident occurred, the speed limit is supposed to be 58 mph. Metro officials would not say how fast the trains were going because of the ongoing NTSB investigation.
President Obama sent condolences to the victims.
"Michelle and I were saddened by the terrible accident in northeast Washington, D.C., today," Obama said in a statement. "Our thoughts and prayers go out to the families and friends affected by this tragedy."
