WASHINGTON -- The deadly train derailment in Philadelphia on Tuesday is raising questions about why Amtrak has not completed installing accident-preventing technology on all 450 miles of track in the Northeast Corridor from Washington to Boston.

The Positive Train Control system is designed to automatically stop or slow a train to avoid derailment caused by excessive speed, collisions with other trains, or entering stretches of tracks by mistake.

In 2008, Congress mandated that Amtrak and most passenger and freight railroads equip their trains and tracks with the technology by the end of this year, though that deadline likely will be extended.

Here are questions and answers about where Amtrak stands in the process and why.

Why did the Frankford curve where the derailment took place lack the Positive Train Control technology?

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Those tracks are among gaps in the system between Washington and New Rochelle, New York, where the technology is not yet fully installed, said Amtrak President Joe Boardman in a news conference Thursday.

According to Amtrak, the system is in place from Perryville, Maryland, to Wilmington, Delaware; New Brunswick to Trenton, New Jersey, and from New Haven, Connecticut, and Boston. Boardman said he's committed to finishing the rest of the Northeast Corridor "by the end of the year."

What has held up installations?

Boardman and other railroad officials say the system has encountered problems with technology, logistics and funding. The system relies on a transponder in the engine connecting with signals on the track, and that communication requires broadband spectrum that railroads had to buy.

"We had to change a lot of things on the corridor to make it work," Boardman said. Radio equipment for the system has encountered interference issues, he said.

What about the money?

"We spent $111 million getting ready for Positive Train Control," Boardman said.

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In an interview with The Philadelphia Inquirer, he added that "with more financing, we could have done it sooner." Amtrak is supposed to be self-sufficient but loses money and is supported by federal funds. Boardman blamed Congress for not appropriating more.

What has Congress appropriated for Amtrak?

Capital funds for Amtrak, which help pay for the safety technology installation, fell from $1 billion in fiscal year 2010 to $902 million in 2013, then rose to $1.05 billion in 2014 and $1.14 billion in 2015, budget records show.

On Wednesday, the Republican-controlled House Appropriations Committee voted to cut the capital funds for fiscal year 2016 that begins Oct. 1 to $850 million.

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How did Democrats react?

Rep. Steve Israel (D-Huntington) charged that Congress "failed" Amtrak riders. He backed a bid to boost Amtrak's funding to the White House request of $2.45 billion. It was voted down.

Why did Republicans make the cuts in funding?

"It's the law," said Rep. Harold Rogers (R-Ky.), the committee chairman, explaining the 2011 Budget Control Act mandates reduced spending.

GOP aides also said their bill left intact $187 million for the Federal Railroad Authority, which is in charge of rail safety, and raised Amtrak's operating subsidy to $288.5 million from $250 million. Those committee-approved funding amounts now must go to the floor for full House approval.

With Sarah Armaghan