A Metro-North train heading from New Haven to New York City derailed around 6:10 p.m. Friday, hitting the other train in Bridgeport, Connecticut, about 60 miles northeast of New York City on the Long Island Sound, authorities said.
That caused some cars on the second train, which was destined for New Haven, to likewise leave the tracks, CNN reported citing a Metro-North spokesperson.
Connecticut Governor Dannel Malloy told reporters Friday night that five people were "critically injured," one of whom was in "very critical condition."
Officials from the National Transportation Safety Board will head the investigation into the crash, with Malloy saying that agency's team should be on site by Saturday morning.
"We have no reason to believe that it's anything but an accident," the governor said.
Power was shut off along the line and service has been halted -- westbound past Bridgeport, and eastbound beyond South Norwalk -- because of the derailment.
Amtrak train service also announced early Friday night that it had suspended all travel between New York and Boston indefinitely after the crash.
Such travel headaches could persist for weeks, because the two tracks affected by the derailment -- which are both "shot right now" -- may take weeks to repair, CNN quoted Bridgeport Mayor Bill Finch as saying.
Because of a bridge replacement project, those two tracks are the only way in and out of New York City by train from that part of Connecticut.
Workers will need to not only remove disabled trains, but also remove the tracks, regrade the rail bed, then lay down the tracks again, according to Finch.
A passenger in a middle car of the New York-bound train, Chris Martin, recalled to CNN how his car went dark after the crash, then someone over the intercom "called all the doctors up front."
Brian Alvarez, who saw the wreckage, described the scene as "pretty graphic."
"I saw this one car and it was completely destroyed, and they were pulling people out of the car," Alvarez was quoted as saying. "... They were all bloody."