About 250 people were on board the Metro-North trains when one heading east from Grand Central Station to New Haven derailed about 6:10 p.m. just outside Bridgeport, MTA and Bridgeport officials said.
The train was hit by a train heading west from New Haven to Grand Central on an adjacent track, MTA spokesman Aaron Donovan said. Some cars on the second train also derailed as a result of the collision.
Investigators did not know what caused the first train to derailment, he said.
"We're most concerned about the injured and ultimately reopening the system," Malloy said from the scene about three hours after the crash. He said there was no reason to believe it was anything other than an accident.
Malloy said that most people were not seriously hurt. Among those critically injured, he said, one's injuries were "very critical."
The Metro-North Railroad, a commuter line serving the northern suburbs, described it as a "major derailment." Photos showed a train car askew on the rails, with its end smashed up and brushing against another train. Amtrak suspended service indefinitely between New York and Boston.
Malloy said there was extensive damage to the train cars and the track, and it could take until Monday for normal service to be restored. He said the accident will have a "big impact on the Northeast Corridor."
The area where the accident happened was already down to two tracks because of repair work, Malloy said. Crews have been working for a long time on the electric lines above the tracks, the power source for the trains. He said Connecticut has an old system and no other alternate tracks.
By late evening, Bridgeport Police Chief Joseph Gaudett said everybody who needed treatment had been attended to, and authorities were beginning to turn their attention to investigating the cause.
"Everybody seemed pretty calm," he said. "Everybody was thankful they didn't get seriously hurt. They were anxious to get home to their families."
The Metropolitan Transportation Authority operates the Metro-North Railroad, the second-largest commuter railroad in the nation. The Metro-North main lines - the Hudson, Harlem, and New Haven - run northward from New York City's Grand Central Terminal into suburban New York and Connecticut.