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 like the train had hit a bump.
"From that point on, it happened so fast, I flew out of the seat and hit my head," she said.
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."
One man said he was riding a bicycle across a bridge over the Metro tracks when the sound of the collision got his attention.
"I didn't see any panic," Barry Student said. "The whole situation was so surreal."
Past accidents on Washington's Metro have included a January 2007 train derailment near the Mt. Vernon Square Station on the green line, in which 23 passengers were transported to local hospitals for treatment and released; a November 2006 accident where a train struck and killed workers in Alexandria, Va.; and a May 2006 accident where a train struck and killed a worker near the Dupont Circle Station.
The Associated Press contributed to this report.