“It’s frustrating always to see where safety recommendations take 50 years to implement. Meanwhile, people are dying,” NTSB board member Jennifer Homendy said. “It is frustrating and heartbreaking. On the other hand, I’m happy where we are today because there will be lives saved in the future.”

The PTC is an automatic braking system designed to take human error out of operating a train. The system automatically applies the brakes if a train is exceeding set speed limits. It can prevent a train from going down the wrong track if a switch is left in the wrong position. And, it can prevent rear-end and head-on collisions by keeping two trains off the same track.

Congress passed legislation mandating the technology after a Metrolink commuter train and a Union Pacific freight train collided head-on at 84 miles per hour, killing 25 people and injuring 135, in Chatsworth, Calif., in 2008.

Railroads were to have the PTC installed by 2015, but the deadline was pushed back twice after the industry complained that it was being rushed to install a complicated technology that was not fully developed. It also argued that the nearly $15 billion cost of installing the systems and training workers on its use was prohibitive. Congress relented and gave railroads until the end of this year to comply.

As the Dec. 31 deadline approaches, the PTC is operating in 98.8 percent of 57,500 route miles that are subject to the 2008 congressional mandate, the Federal Railroad Administration (FRA) said in a progress report last month. The technology has yet to be activated on 700 route miles as of June 30, the agency said.

The congressional mandate applies only to large freight railroads’ mainlines that carry toxic hazardous materials and any mainlines carrying intercity or commuter passengers. That’s why only about 58,000 of the roughly 140,000 routes miles that make up the nation’s rail network are required to be PTC-ready.

Of the 41 commuter railroads subject to the mandate, two — New Jersey Transit and New Mexico Rail Runner — are behind in implementation and at risk of missing the December deadline, the FRA said. The other 39, including Amtrak and other commuter systems such as Maryland Area Regional Commuter Train Service and Virginia Railway Express, have the system in service or at various levels of testing, according to railroad reports.

An FRA spokeswoman said the agency is directing additional resources and technical assistance, including on-site support, to the New Jersey and New Mexico systems, guiding them on the remaining actions they need to take to fully implement the PTC by the year-end deadline. Amtrak said it has completed PTC implementation on the 898 route miles it owns, and that all 550 locomotives and all radio towers are equipped with the system. PTC training is also completed, the company said.

“Amtrak is pleased to achieve this milestone, and we will continue to work together with all of our partners to improve safety across the rail network,” Amtrak Executive Vice President and Chief Safety Officer Steve Predmore said in a statement.

Amtrak’s announcement, however, refers only to its assets, which are a small share of the track that the company uses. Officials with the passenger railroad said they are working with host railroads to ensure its system works flawlessly on the other 20,000 route miles it uses nationwide. As of last month, just over 98 percent of the more than 20,700 miles on host-controlled track operate with the PTC, Amtrak said.

Ian Jefferies, president of the Association of American Railroads, said railroads have invested $11.5 billion in the PTC. He said the Class 1, or big freight railroads, are 100 percent operational on all their PTC route miles.

“All of our training is completed 100 percent. The locomotives are equipped 100 percent. The spectrum required is in place,” Jefferies said.

The railroads are finalizing and working through ensuring that their systems are interoperable, he said. Because railroads operate over each another’s tracks, and one railroad locomotive can be on another railroad’s track on any given day, all railroads’ PTC systems must communicate with each other.

“Railroads have been extremely diligent and thorough in development testing and validation of their systems,” Jefferies said. “It has been a long, herculean effort.”

Industry and rail safety leaders say they are encouraged by the railroads’ progress to meet this year’s deadline, citing only concerns about possible holdups among the two commuter railroads.

The NTSB first issued a recommendation for the PTC in 1969, following the collision of two commuter trains in Darien, Conn., that killed four people and injured 43. The safety panel has since urged railroads to invest in the technology and pleaded with federal regulators to mandate it. The PTC was on the panel’s “most wanted list” for decades.

NTSB records show that more than 300 people have been killed and nearly 7,000 injured in 154 crashes that could have been prevented by the PTC.

But experts also warn that the PTC won’t prevent all train crashes, such as collisions with vehicles at grade crossings or those caused by track and equipment failures.

Positive train control guarantees that trains will round curves at a safe speed, preventing derailments due to overspeeding like the 2017 Amtrak crash near Dupont, Wash., that killed three people and injured 62. Investigators concluded that the lack of the automatic braking system allowed the engineer to enter a 30 mph curve at 80 mph.

In a similar Amtrak derailment in 2015, an engineer entered a curve in Philadelphia at twice the posted speed limit. Eight people were killed, and 159 injured. Both crashes could have been prevented with the PTC, the NTSB concluded.

“I am highly pleased by the amount of progress railroads have made moving toward fully implementing PTC systems,” FRA Administrator Ronald L. Batory said in last month’s progress report.

The FRA said it contributed $3.4 billion in grants and loans to support PTC implementation, and the agency is providing technical assistance to get the systems rolling.

Homendy, the NTSB board member, said she anticipates some bumps in the system as it launches and glitches in the software over time that will need to be addressed, as is the case with most technology.

“It’s not like Dec. 31 comes and that’s the end,” Homendy said. “It’ll be a constantly evolving technology that needs to be constantly implemented, just like any technology out there. There will be additional work.”

But, she added, “it will save lives.”