A half-century after a federal safety board envisioned an automatic train braking system to prevent major train collisions — and a dozen years after Congress ordered its installation — transportation officials say the project has been completed.

The Federal Railroad Administration announced Tuesday that positive train control safeguards are in operation on nearly 57,500 required miles of the nation’s railroads. It came two days before a Dec. 31 deadline.

The positive train control, or PTC, braking system automatically slows a train if it is exceeding set speed limits. The system also can prevent trains from going down the wrong tracks if switches are in the wrong position and can prevent collisions by keeping two trains off the same track.

Federal officials called the system the biggest safety advancement in railroads since the use of signals.

“Achieving 100 percent PTC implementation is a tremendous accomplishment and reflects the Department’s top priorities — safety, innovation and infrastructure,” Transportation Secretary Elaine Chao said in a statement.

Operated by train and track operators, the PTC systems involve controls aboard trains and along the tracks. They share the same technical requirements, allowing for coordination between railroads. The system monitors trains using GPS.

“This is a huge deal,” said Jennifer Homendy, a National Transportation Safety Board member. “For the rail industry, this is the biggest thing since the modern signal system was created. For us, it was a huge endeavor.”

Homendy said the NTSB began focusing on creating such a system about 50 years ago.

Since 1969, the safety board has investigated 154 rail incidents that resulted in 305 fatalities and 6,885 injuries that could have been prevented with PTC, she said.

Congress mandated the system in the Rail Safety Improvement Act of 2008, spurred by train collisions and incidents involving hazardous chemicals in 2005 and 2006, as well as a deadly head-on collision of a Metrolink commuter train and a Union Pacific freight train on Sept. 12, 2008, in Los Angeles.

Railroads initially were given until 2015 to install PTC, but the deadline was extended twice when the industry complained it was being rushed into installing a technology that was not fully developed. Railroad officials also said training workers to use the system was expensive and time-consuming.

The nation’s Class I railroads, the largest, invested nearly $11.5 billion in the system, the Association of American Railroads said. The Transportation Department provided $3.4 billion in grant and loan funding for the project, according to the Federal Railroad Administration.

“America’s railroads have reached an important milestone this year that will enhance safety and springboard innovation long into the future,” Ian Jefferies, the president and chief executive of the AAR, said in a statement. “While the industry is proud of this accomplishment, the job is never finished. Railroads will remain forward-looking and continue advancing safety through innovation and technology.”

Forty-one railroads were required to install PTC systems — seven Class I railroads, Amtrak, 28 commuter railroads and five freight railroads that regularly host commuter passenger service.

Nearly 100 host and tenant railroads, associations, service providers and suppliers were involved in the project, the Federal Railroad Administration said.

“At its core, PTC is a risk-reduction system that will make a safe industry even safer, and provide a solid foundation upon which additional safety improvements will be realized,” Federal Railroad Administration Administrator Ronald L. Batory said in a statement.

The Rail Safety Improvement Act required PTC systems on Class I railroad mainlines that carry 5 million or more gross tons of annual traffic and certain toxic hazardous materials, as well as on any mainlines over which intercity or commuter passenger transportation occurs. The roughly 57,500 miles equipped with the system are just a portion of the nation’s roughly 140,000 miles of rail network.

Homendy said the installation of the system is the first step, with federal officials hoping to expand and fine-tune the system to provide greater control of trains.