(CNN) -- Nine 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 busy corridor from New York to New Haven, Connecticut, are focusing on a broken rail as a possible cause behind Friday's rush-hour collision.
"It is of substantial interest to us, and we will be sending a portion of that track back to the laboratory in Washington, D.C., for analysis," Earl Weener of the NTSB told reporters Saturday.
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 -- both closed indefinitely.
Investigators have ruled out foul play in the crash, which left more than 70 people injured.
John Cappiello, a spokesman at Bridgeport Hospital in Connecticut, said three patients were still there Sunday -- one person in critical condition and two listed as stable. Six patients remained at St. Vincent's Medical Center in Bridgeport, all in good condition, spokeswoman Lucinda Ames said.
The Metropolitan Transportation Authority said Sunday that 13 of the 16 cars had been removed from the accident site and the others should be cleared by the end of the day.
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 crew.
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 rail 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.
'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, officials said.
"Ribbons of the sides of cars are torn away like ribbons of cloth," Blumenthal 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.
'Doctors up front'
A passenger in a middle car of the New York-bound train, Chris Martin, said his car went dark after the crash.
He then heard someone yell over the intercom for "all the doctors up front."
Martin said his crowded train was evacuated. Everybody on his train was physically fine, he said, but many were shaken emotionally. He said he saw injured people outside the train.
Brian Alvarez viewed the wreckage.
"I saw this one car and it was completely destroyed, and they were pulling people out of the car," Alvarez said. "... They were all bloody."
CNN's Susan Candiotti, Chris Welch, Rob Frehse, Chris Welch, AnneClaire Stapleton, Jason Hanna and Jordana Ossad contributed to this report.