Hundreds of people were on the trains involved in Friday evening's crash just outside Bridgeport, Connecticut.
Officials said a train that left New York City's Grand Central en route to New Haven, Connecticut, derailed then was hit by another train.
Amtrak has suspended its service between New York and Boston.
Connecticut Governor Dannel Malloy said on Friday night that the front of one of the trains had been extensively damaged and its wheels were "sticking into the other train".
He said five people had received serious injuries, describing one person as being in a "very critical" condition.
The two hospitals in Bridgeport that received the wounded said they were each treating one person in a critical condition. By Saturday morning, many of injured passengers had been released from hospital.
Investigators are trying to find out what caused the crash, which happened shortly after 18:00 local time (22:00 GMT).
Governor Malloy said he had no reason to believe it was anything other than an accident.
Bridgeport Police Chief Joseph Gaudett said most of those hurt were walking wounded.
"Everybody seemed pretty calm," he told the Associated Press. "Everybody was thankful they didn't get seriously hurt. They were anxious to get home to their families."
But Canadian passenger Alex Cohen told NBC Connecticut that "people were screaming... they had to smash a window to get us out".
One witness, Brian Alvarez, told CNN: "I saw this one car and it was completely destroyed and they were pulling people out of the car. They were all bloody."
More than 60 people were injured, at least two critically, after a head-on, rush-hour collision between two commuter trains near New York City.