Metropolitan Transportation Authority spokeswoman Marjorie Anders said the westbound 4:41 p.m. Metro-North commuter train out of New Haven derailed near the Fairfield station at 6:10 p.m., hitting the side of an eastbound train on the adjacent track and causing some cars on that train also to derail.
Police said four of the 22 people who were taken to the hospital had serious injuries, The Hartford Courant reported.
Passengers were removed from both trains and bused to other stations.
All Metro-North and Amtrak service in Connecticut was suspended, and repairs could take several days, the newspaper said.
Amtrak announced all service between New York and Boston was suspended indefinitely.
Bridgeport Mayor Bill Finch described a "scary" scene, saying, "We dodged a bullet" in that no fatalities were reported.
But he said the extensive damage to the tracks could stymie commuter service indefinitely.
"We're in a pretty precarious situation here," he said.