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(CNN) -- Five people remained in the hospital -- one in critical condition -- two days after a commuter train derailed and struck another train on one of the busiest tracks in the country, officials said Sunday.
National Transportation Safety Board investigators looking at the accident, along the corridor from New York to New Haven, Connecticut, are focusing on a broken rail as a possible cause for Friday's rush-hour collision.
"All of our teams have been working around the clock to gather information and facts that will help us determine what caused this accident and what we can do to prevent it from happening again, Earl Weener of the NTSB told reporters Sunday.
For now, a long stretch of track that more than 30,000 passengers use daily will be shut down. That includes Metro-North service for a 30-mile stretch between New Haven and South Norwalk, Connecticut, and Amtrak service between New York and New Haven.
Amtrak released a statement Sunday night saying that service between Boston and New Haven also would be limited, and "there is no estimate on service restoration." Because of that inconvenience, Amtrak said passengers who "have paid but choose not to travel due to this service disruption can receive a refund or a voucher for future travel."
Investigators have ruled out foul play in the crash, which injured more than 70 people.
Cindy Nunes and John Cappiello, spokespeople for Bridgeport Hospital in Connecticut, said three patients were still there Sunday -- one person in critical condition and two listed as stable. Two patients remained at St. Vincent's Medical Center in Bridgeport, both in good condition, spokeswoman Lucinda Ames said.
By Sunday afternoon, all the rail cars had been removed from the accident site, said Weener. Two segments of rail in the area of the fracture also had been removed and are being sent to a laboratory for analysis, he said.
None of the cars flipped over when the two trains collided, but many cars were heavily damaged. Some had gaping holes where doors had been. Deep scrape marks were easily visible where one train sideswiped the other.
Investigators will look at the trains' braking performances, wheel and track conditions, and speed and other information from data recorders, Weener said. In addition to the trains, investigators also are examining the actions of the crews.
He said the track could have been broken by the accident or could have been fractured before the trains collided.
This accident involved commuter rail cars built to new codes, he said Sunday.
"This gives us a chance to see how effective the new standards are," he told CNN.
The two tracks will have to be repaired before they can be reopened.
"Our crews will essentially be rebuilding 2,000 feet of damaged track and overhead wires and signal system," Metro-North Railroad President Howard Permut said in a statement.
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.
Connecticut Gov. Dannel Malloy said the state would set up a system to take Metro-North patrons from Bridgeport to the closest operable station, in South Norwalk, during the shutdown.
The city of Fairfield, which has three affected train stations, sent out a message telling commuters there will not be enough extra buses to handle normal passenger volume on Monday. It encouraged people to consider staying home.
'Everybody was doing what they could'
Daniel Solomon, a trauma surgeon, was in the front car and barely felt the crash when it happened.
He didn't understand the severity of the situation at first.
"But when I reached the back of the car, I realized that the damage was a lot more severe, and people were filing out of the middle car pretty badly bloodied," he told CNN on Sunday.
His medical training kicked in as he helped an injured woman off the tracks and attended to others. Solomon said his fellow commuters remained calm and were eager to lend a hand.
"I think everybody's Good Samaritan instinct took effect, and everybody was doing what they could," he said.
'Absolutely staggering' damage
The damage to the tracks and several train cars is "absolutely staggering," said Sen. Richard Blumenthal of Connecticut, who visited the site with other officials Saturday. Wreckage littered an area of about 200 yards.
"Ribbons of the sides of cars are torn away like ribbons of cloth," the senator said. "Tons of metal tossed around like toy things. The insides of cars are shattered."
The two Metro-North passenger trains, heading in opposite directions, collided Friday evening in southwestern Connecticut. The train heading from New York City to New Haven derailed around 6:10 p.m. and struck the other train in Bridgeport, Connecticut, Weener said Saturday.
Both trains were traveling at about 70 mph immediately before the crash.
CNN's Chelsea J. Carter, Susan Candiotti, Chris Welch, Rob Frehse, Chris Welch, AnneClaire Stapleton, Jason Hanna, Jordana Ossad, and Mark Morgenstein contributed to this report.