Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

PHILADELPHIA AMTRAK CRASH: THE LATEST

A new safety system would have prevented the accident, the NSTB says.

The train was traveling at 106 mph, the NTSB confirms.

Authorities still have not accounted for everyone on board.

Eight passengers critical because they "rattled around in the train car a lot."

BY M. ALEX JOHNSON, TOM COSTELLO, RICHARD ESPOSITO AND ERIN McCLAM

The stretch of track where Amtrak Regional 188 derailed Tuesday night wasn't equipped with a new safety system that Congress has mandated be implemented by the end of the year, so when the train hurtled around a curve in Philadelphia at twice the speed limit, it jumped the tracks rather than automatically being brought to a safe stop, federal safety investigators said Wednesday.

Authorities raised the death toll to seven in the crash Tuesday night and said they still hadn't accounted for all of the 243 people who were on board.

The train was traveling at 106 mph as it entered a curve where the speed limit is just 50 mph, Robert Sumwalt, a member of the National Transportation Safety Board, said late Wednesday afternoon. The entire train — a locomotive and seven passenger cars — derailed immediately, he said.

RELATED: Railroad Crashes Are Up, and Amtrak Is Up Even More

But that didn't have to happen, Sumwalt said.

Amtrak, like all other U.S. railroads, is required by law to implement a safety system called Positive Train Control, or PTC — designed to automatically slow trains and enforce speed limits — by the end of the year.

Tracks in the Northeast Corridor are being updated — but crews hadn't yet gotten to that stretch of track yet, Sumwalt said.

The NTSB has lobbied vigorously for PTC since 1970, declaring it one of the agency's top priorities, and Sumwalt said Wednesday: "Had such a system been installed in this section of track, this accident would not have occurred."

But a bipartisan group of U.S. senators is seeking to delay the PTC mandate until at least 2020 at the behest of railroads that have asked for more time — which the NTSB said in its list of top requests for 2015 (PDF) means more preventable collisions and derailments, "more lives lost and more people [sustaining] injuries that change their lives forever."

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

"Each death, each injury, and each accident that PTC could have prevented, testifies to the vital importance of implementing PTC now," the agency said.

A city official said hospitals treated more than 200 patients. The train was believed to have carried 238 passengers and five crew members, meaning it would have been about half full. Sam Phillips, the city's emergency management director, asked people who walked away from the crash to call Amtrak at 800-523-9101 so they could be accounted for.

The seven dead comprised four people whose bodies were found inside the train, two who were found outside and one who died at a hospital, police Lt. John Walker told NBC Philadelphia.

Four were identified:

Rachel Jacobs, chief executive of ApprenNet, a Philadelphia education technology firm.

Justin Zemser, 20, a Navy midshipman who was on leave and going home to New York.

Jim Gaines, 48, an Associated Press photographer and video software architect.

Abid Gilani, a senior vice president in the commercial real estate division for Wells Fargo.

Derrick Griffith, educator who founded a program to help out-of-school youth.

"We've suffered a tragedy here in our city," Mayor Michael Nutter said at an afternoon news conference. "I don't believe that anyone standing here today has any memory of a derailment of this kind in 50 years."

Dozens of people were still being treated in Philadelphia hospitals with injuries ranging from cuts and broken bones to head trauma.

Chief Medical Officer Herbert Cushing said Temple University Hospital, where many of the most seriously injured were being treated, had eight patients in critical condition, who he said "are going to do just fine."

"Almost everyone has rib fractures," Cushing said, which indicates that "they rattled around in the train car a lot."

All of the patients at Temple are adults ranging in age from their early 20s to their 80s, Cushing said. Patients from Spain, Belgium, Germany, India and Albania were among those involved, he said.

The engineer, identified as Brandon Bostian, was released from Einstein Medical Center and was questioned by police, Philadelphia officials said.

Investigators spent the day combing for clues at what Sen. Pat Toomey, R-Pennsylvania, described as "a horrific and heartbreaking scene." The mayor vowed not to call off the search until crews had checked out every inch of the wreckage.

The derailment damaged all seven passenger cars, including some that were overturned and one that was mangled. Passengers and luggage were tossed around inside, and survivors described having to force doors open or clamber through windows to safety.

Grainy security footage from a nearby camera captured several flashes of bright light as the train crashed.

Sumwalt of the NTSB said investigators would look at a range of factors, including track conditions, signals, mechanics and human performance.

READ MORE: Full coverage from NBC 10 Philadelphia

Amtrak suspended service between New York and Philadelphia on its Northeast Corridor, the busiest stretch of track in the country for passenger travel. The section of track where the train derailed will have to be rebuilt, Sumwalt said.

Passengers described a sudden shake, then a harrowing scene.

"Chairs inside the train became unscrewed, and suitcases were falling on people," said Max Helfman, 19, who was returning home to New Jersey when the train car he was in flipped over.

"My mother flew into me, and I literally had to catch her," he said. "People were bleeding from their head. It was awful."

IN-DEPTH

Jay Blackman, Hasani Gittens, Phil Helsel, Tom Winter, Shamar Walters, Tony Dokoupil, Ben Popken, Aliza Nadi and Jonathan Dienst of NBC News contributed to this report.