WASHINGTON - Members of Congress pointedly questioned federal rail investigators Tuesday over why they still don't know whether the engineer operating Amtrak Train 188 was using his cellphone when it crashed in Philadelphia on May 12.

Three weeks later, "I just don't understand what the holdup is," Rep. Barbara Comstock (R., Va.) said at the first congressional hearing into the derailment that killed eight people and injured more than 200.

Adding to lawmakers' frustration was that the National Transportation Safety Board has access to the engineer's cellphone and password, but has not nailed down an answer.

"It seems," said Rep. Lee Zeldin (R., N.Y.), "if you get the access to the phone, you look at the phone, you know the answer in, like, five minutes."

"We were surprised by the complexity of it ourselves," responded the safety board's chairman, Christopher Hart, "and we're experts at this."

Hart told the House transportation committee that while investigators know that the phone was used on the day of the crash, the precise timing of that use is not yet clear because of discrepancies among various sources of information. He could not say whether the phone was on or off at the time of the accident.

The time stamps on the phone's texts and calls, for example, are in different time zones, Hart said (the phone had a California number). He said there are some discrepancies between times on the phone and those in cellphone company records.

Investigators are also trying to match the phone times with other records, such as surveillance cameras, a train camera, and radio communications, he said.

"We want to have exact agreement on this before we come out with any of it," Hart said.

Investigators are looking at differences of less than a minute, he said, but in the context of a train going more than 100 m.p.h., seconds could make a difference. "We want to pin it down to the microsecond," he said.

The question about the cellphones touches on one of the crash's mysteries: Why was the train going so fast as it approached a curve where the speed limit was 50 m.p.h.?

The NTSB said Tuesday that Amtrak estimates the derailment cost in excess of $9.2 million.

Hart said there were no anomalies with the track, brakes, or locomotive, pointing to an as-yet-unspecified human error.

"There are very few answers right now, three weeks after one of the most horrific crashes that our nation has ever seen," Rep. Jeff Denhem (R., Calif.) said, frustration in his voice.

Investigators will eventually find answers, promised Sarah Feinberg, acting administrator of the Federal Rail Administration. Hart could not say when his NTSB team expects to reach a conclusion regarding the phone.

The engineer of Amtrak 188, Brandon Bostian, 32, has told investigators he does not remember anything about the crash. His lawyer has said that the cellphone was put away in a bag.

Bostian, who said he suffered a concussion in the crash, has cooperated with the NTSB but has declined to be interviewed by police.

Much of the hearing focused on safety improvements that could have prevented the wreck or similar accidents.

All sides of the passenger rail industry - Amtrak, federal regulators, safety officials, and the labor union representing engineers - stressed the importance of Positive Train Control, an upgraded safety system that could have stopped or slowed the train before it derailed. "We need technology that can step in when humans fail," Hart said.

"The single greatest contribution that my generation of railroaders can make to the industry is to implement [Positive Train Control] as rapidly as possible," Amtrak chief executive Joseph Boardman told the committee.

But while Congress has required all major freight and passenger rail lines to have the system in place by year's end, few expect to meet that deadline.

Democrats said more investments in infrastructure could have helped, but Republicans accused rail lines of squandering their resources.

"Of all the people who should be apologizing . . . the United States Congress ought to be on the top of that list. The safety of the American people has been compromised because we have been dithering," said Rep. Sean Patrick Maloney (D., N.Y.).

But several Republicans accused Amtrak of wasting the money it has - about $1.4 billion a year from Congress. Rep. Scott Perry (R., Pa.) asked why money from the $800 billion federal stimulus passed in 2009 wasn't used to install Positive Train Control, "if it's such a priority."

Some Democrats questioned why seat belts are not mandated on trains, as they are on planes.

"I don't understand why, in 2015, we're still analyzing this," said Rep. Albio Sires (D., N.J.).

The safety board is examining the issue, Hart said.

Feinberg said that installing seat belts would require hardening train seats, which could prove dangerous for passengers who are out of their seats.

Other lawmakers pressed for railroads to install "inward-facing" cameras focused on the engineers, to help investigations.

Amtrak took that step last week, though the labor union representing engineers has raised concerns about the cameras' use, citing privacy.

Hart said the safety board is studying passenger cars to see if they can be improved to better withstand crashes, while Feinberg said the FRA is mulling a package of recommendations that could address training, distractions, and other human factors that contribute to wrecks.

Boardman explained to lawmakers why an older safety system was in place on the southbound side of the tracks near Philadelphia - where the speed limit is higher - but not the northbound side, where the train derailed.

The speed limit approaching the curve going northbound is 80 m.p.h., and a train could navigate the curve at just over 90 m.p.h., Boardman said. The train was traveling at 106 before the engineer hit the emergency brake.

"The notion that an engineer might actually accelerate into the northbound curve was not a circumstance we anticipated," Boardman said in his opening statement.

The speed limit on the southbound side is higher, with the older safety system in place to ensure engineers slow down.

Since the accident, the safety system has been added to the northbound side, as ordered by the Federal Railroad Administration.

Boardman pointed out that Amtrak will meet the deadline, despite any Democratic complaints about funding.

With Congress considering extending the deadline, all four witnesses who spoke Tuesday said the system could prove critical to safety.

The system has been stalled by an array of snags - trouble acquiring the radio spectrum needed to operate it, regulatory hurdles, and the cost for public commuter agencies with limited budgets among them - but lawmakers differed sharply on whether more federal funding could have made a difference.

Feinberg pointed in her written testimony to long-standing requests for more funding to assist with the implementation of Positive Train Control.

Boardman said that only $1.3 billion of the stimulus went to Amtrak and that Congress had instructed it to spend it on infrastructure projects "ready to go." He said that while Amtrak is on pace to meet this year's deadline, more money earlier might have sped up the process.