The second train, carrying about 70 passengers, had pulled into one of the city's busiest stations moments before the empty train backed into it. Most of the 75-foot-long rear car of the empty train came off the tracks and about one-third of its aluminum shell was pulled apart.
Witnesses said the operator of the occupied train began screaming for people to get off a few seconds before impact.
"The next thing we knew there was a big crash, and a lot of dirt and dust flying everywhere, and panic, and everybody just ran out of the Metro station," said Mike Cucciardl, a teacher traveling on the train with more than 40 kids from a District of Columbia charter school. Thirteen students were among the injured.
None of the injuries was life-threatening, said Alan Etter, a fire department spokesman. He said it was "astounding," given the force of the crash, that more people weren't more seriously hurt. The worst injury appeared to be a broken leg, he said.
The crash happened after a loaded six-car train pulled into the Woodley Park-National Zoo Metro station around 12:49 p.m., spokesman Steven Taubenkibel said.
Each train has an automatic control system to keep trains a safe distance from each other, Deputy General Manager for Operations James Gallagher said. Officials were looking into whether the system failed.
"The train rolled backward. Trains don't roll backward," Gallagher said. "There's something unusual about this event."
Train operators can manually override the automatic braking system, but officials said that would require permission from the operations control center. Officials said they were questioning the two drivers and examining mechanical records. Metro officials didn't believe the crash was related to terrorism.
Metro called Calvert Sawyers, the 58-year-old train operator who warned passengers of the oncoming train, a life saver.
"It could have been a horrible disaster, and he was one of the reasons it wasn't," Metro spokesman Ray Feldmann said.
The National Transportation Safety Board took control of the scene late Wednesday afternoon. Metro officials said the trains won't be moved until the NTSB completes its probe.
Investigators said they would look at the age of the trains, their braking systems, maintenance records and the possibility of operator error. Neither train had an event recorder, or "black box," an NTSB spokesman said. However, investigators were hoping to get information about the crash from station surveillance video.
The Washington subway system's last serious crash was in January 1996. An operator was killed after his train slid past an outdoor platform and collided with an out-of-service train during a winter snowstorm.
Wednesday's crash disrupted service on the Metro's Red Line for nearly four hours. Eventually, trains were able to run on a single track through the Woodley Park station, which will remain closed for at least another day.
Metro made the usual move of bringing in eight-car trains and reducing its fares for the entire system in an effort to ease the difficult evening commute. Dozens of buses shuttled commuters between the affected stations, although some people became frustrated by the long wait and opted to walk.