At least two of the cars had flipped on their side after the crash, and one car stopped only feet from the banks of the Harlem River. Cuomo told reporters that the four fatalities were from the two cars that had flipped on their side. Firefighters, rescue workers and ambulances responded quickly to the scene, and some firefighters broke through windows to remove passengers from the train.
Some of the passengers were jolted from their sleep to screams and the frightening sensation of the train car rolling over.
"It was the speed of the car when I woke up and felt it tilting a bit and by the time I realized what was going on, everyone was getting thrown around," passenger Ryan Kelly told ABC News.
Kelly survived the crash but suffered a fractured hand.
"Everyone in my car was alright except for one woman who was pinned between two seats," he added.
ABC News' Aaron Katersky, Matt Hosford, Howard Price and Gio Benitez contributed to this report.