With new safety measures in place, passenger trains resumed shuttling between Philadelphia and New York City on Monday, restoring full service to the bustling Northeast Corridor for the first time since last week's deadly Amtrak derailment at Frankford Junction.

Amtrak spokesman Craig Schulz said the railroad installed an automatic train-control system over the weekend on the northbound tracks to limit speed approaching the curve to 45 m.p.h.

Train 188 was traveling at 106 m.p.h. Tuesday when it derailed, killing eight people and injuring more than 200.

As routine returned to the rails Monday, authorities were still trying to determine a number of facts about the cause of the crash.

Three passengers were buried Monday, and as three new lawsuits were filed against Amtrak, legislation was introduced in Washington that could raise the maximum payout in such incidents from $200 million to $500 million.

The National Transportation Safety Board, which is continuing its investigation in conjunction with Philadelphia police, said it expects to release an update by midweek.

"There's a lot of questions we want answers to," said NTSB spokesman Peter Knudson.

In advance of its report, the NTSB announced on Twitter that the FBI had determined that a firearm could not have caused the damage to the train's windshield. The NTSB did not rule out the possibility that another object could have struck the glass, according to the agency.

Funerals for the three passengers were Monday in Michigan, New York, and New Jersey.

"My sister was my role model, my confidant, and my best friend," Jessica Steinhart said of Rachel Jacobs, 39, who had recently become CEO of a University City education-technology start-up, the Detroit News reported.

About 1,500 people attended the Swarthmore College graduate's service in Southfield, Mich.

The family of one crash victim, Italian businessman Giuseppe Piras, has been trying to cut through red tape to secure his remains after they were unexpectedly transported to New Jersey.

"A very difficult situation," the Italian consul general in Philadelphia said.

At least 16 patients were still hospitalized for their injuries, with six in critical condition, hospital officials said.

One of the lawsuits was filed by two Philadelphia lawyers on behalf of four passengers. The suit contends the rail line had failed to install an automatic-braking system on the curve where the train left the tracks.

The NTSB spokesman said investigators were also seeking documents related to engineer Brandon Bostian, 32. They are interested in the New York-based engineer's training records, cellphone records, and schedule on the day of the crash, Knudson said.

The NTSB had also not yet determined why an assistant conductor reported hearing Bostian tell a SEPTA engineer that his train had been struck with a projectile moments before the crash, Knudson said.

The agency had reviewed taped conversations between Bostian and dispatchers, Knudson said, and "did not hear" Bostian report anything hitting his train.

"What she thought she heard, we don't have" on tape, Knudson said. He also noted that any radio chatter from Bostian would have been made on one frequency and therefore recorded.

The NTSB had not talked to the assistant conductor a second time, he added.

At an afternoon news conference in Center City, SEPTA Police Chief Thomas Nestel said the agency's trains are regularly struck by projectiles on the tracks it shares with Amtrak.

Yet "in the three years I've been here," he said, "I can't think of an incident where we've had injuries or a serious service interruption" as a result.

One of the passengers who sued Monday, Felicidad Redondo Iban, a Spanish tourist, was pinned under a car and has had multiple surgeries to save a nearly severed right arm from amputation.

"It is utterly inexplicable to me that someone would accelerate into a sharp curve, unless they were not paying attention or that they believed they had already gone through the curve and were accelerating into a straightaway," said personal injury lawyer, Robert Mongeluzzi, who filed the suit.

Two other claims were filed Monday, one by a Swiss couple in federal court in Manhattan and another by a conductor, Emilio Fonseca, in state court in Newark, N.J.

In an interview Monday, Mongeluzzi said Amtrak's installation of the new technology over the weekend shows that it could have made the lines safe all along but chose for financial reasons not to.

"They had the absolute ability to do it in a short period," he said.

As the suits were filed, Sen. Bill Nelson of Florida, the ranking Democrat on the Transportation Committee, introduced legislation that would raise the cap on payouts in Amtrak accidents from $200 million to $500 million. Congress enacted the cap in 1997 to help stabilize Amtrak financially, as part of a larger restructuring package that called for weaning the rail line from federal support.

Litigation experts say that with eight dead and hundreds injured, it is likely that claims against Amtrak will far exceed $200 million.

"We can't allow anyone to suffer additionally due to an outdated cap based on mid-1990 dollars," Nelson said.

The family of Piras, an olive oil entrepreneur, was suffering in a different way.

According to Andrea Canepari, the consul general of Italy in Philadelphia, Piras' brother arrived in the Philadelphia area late last week to return his brother's remains to Sardinia.

But the remains had been transported to a funeral home in North Jersey at the request of an insurance company, Canepari said.

Canepari said the death certificate and other paperwork must now be handled by two states, causing more delays. The consul general said he believes arrangements will be made soon to have the body sent to Italy. The late passenger's brother, Antonia, remains here, hoping to accompany his brother's body home.

In Queens, New York City, a funeral was held for Laura Finamore, 47, a managing director of Cushman & Wakefield, according to CBS New York. In a statement, her family said Finamore's "infectious laughter will be remembered by many for years to come," the station reported.

And in Holmdel, N.J., Bob Gildersleeve, 45, was remembered by his family as a loving and caring father.

"I'm so lucky to have gotten to travel all over the world with you," said his daughter, Ryan, 16, according to NJ.com. "No matter where we were, the canals in Venice, the pyramids in Egypt, or the aisles of Home Depot, you made every trip fun."

cpalmer@phillynews.com

609-217-8305 @cs_palmer

Inquirer staff writers Melanie Burney, Matt Gelb, and Aubrey Whelan contributed to this article.