– Four of the nation’s commuter railroads won’t meet an end-of-next-year deadline to fully implement a collision-avoidance system that Congress required nearly a decade ago.

Three of the country’s largest freight railroads will not be able to finish their systems until 2020, according to reports filed this month with the Federal Railroad Administration.

That’s despite lawmakers extending the original deadline for completing positive train control, originally December 2015 to December 2018. A spending bill lawmakers approved earlier this month included $199 million in funding to help the commuter railroads get the equipment installed on locomotives and track, and to train ­employees.

The system automatically slows or stops trains to prevent collisions and to prevent trains from taking curves too fast. It also slows or stops trains when railroad workers are present.

“It’s moving incredibly slowly,” said Sarah Feinberg, who was a Federal Railroad Administration chief in the Obama administration. “That money should be going out the door.”

While the railroads expressed confidence in meeting the requirements, Sen. Richard Blumenthal, D-Conn., one of the industry’s sharpest critics, said he was “deeply concerned that we’re going to see déjà vu all over again.”

In the months leading up to the original 2015 deadline, the railroads threatened to shut down until Congress granted the extension.

“The history here is that the railroads seek one delay after another,” Blumenthal said.

Railroads have cited technological and legal complexities for the delays in rolling out the technology on 60,000 miles of track. Freight carriers have also cited cost as a factor. To date, they’ve spent about $8 billion of their own funds on PTC deployment.

Passenger railroads will wind up spending an estimated $3.5 billion on their systems. They’re getting help from the federal government in the form of grants and loans.

Most railroads will have their PTC systems complete by the end of next year. Six railroads, however, won’t have PTC fully implemented until 2020 — the three freight railroads and three commuter lines:

• Canadian National, a large Canadian freight carrier with extensive operations in the central U.S.

• CSX, the largest freight carrier in the eastern U.S. and host railroad for Amtrak and commuter lines.

• Norfolk Southern, the primary rival to CSX in the East and also a host of Amtrak and commuter trains.

• Chicago’s Metra, the country’s fourth largest commuter-rail system.

• Boston’s Massachusetts Bay Transportation Authority, the country’s sixth largest commuter-rail system.

• Florida’s SunRail, a commuter line that serves the Orlando area.

A collision between a Metrolink commuter train and a Union Pacific freight train in Southern California in 2008 that killed 25 people was the impetus for Congress to pass the Rail Safety Improvement Act, the law that required the safety system. The National Transportation Safety Board had been calling for such a system since 1970.

Additional crashes since Congress required positive train control have only increased the sense of urgency for installing it.

An Amtrak train derailed in Philadelphia in 2015, killing eight passengers. The train had sped into a 50 mph curve at more than 100 mph.

In the Bronx in 2013, a Metro-North Commuter train derailed on a 30 mph curve at 82 mph. Four passengers were killed.

“We cannot have delays getting commuter railroads the money they need,” ­Feinberg said.