OMAHA, Neb. (AP) — The automatic braking system railroads were required to install several years ago needs improvement to better prevent collisions, federal safety investigators said in a report Wednesday.

The National Transportation Safety Board’s report urged the Federal Railroad Administration and the industry to keep developing new technology that can be used to improve Positive Train Control systems. Their recommendations included at least one practical idea that railroads could likely implement relatively quickly.

Railroads spent 12 years and roughly $15 billion to develop and install the automatic braking system after Congress required it in 2008 in the wake of a collision between a commuter and freight train in California that killed 25 and injured more than 100.

The system, in place on about 58,000 miles (93,000 kilometers) of track nationwide since 2020, is designed to reduce human error by automatically stopping trains in certain situations, such as when they’re in danger of colliding, derailing because of excessive speed, entering tracks under maintenance or traveling the wrong direction.

The National Transportation Safety Board has said more than 150 train crashes since 1969 could have been prevented by Positive Train Control. The agency had recommended the automatic braking system for years before it was mandated by Congress, which extended the original 2015 deadline twice and gave railroads until the end of 2020 to complete the system.

The Association of American Railroads trade group said the industry is focused on “maintaining and advancing” the braking system and will keep looking for improvements.

“Railroads continue their work to enhance the system in ways that further improve safety and drive down accidents,” association spokesperson Jessica Kahanek said.

The Federal Railroad Administration didn’t immediately comment on the new report.

Railroad safety has been a key concern nationwide this year ever since a Norfolk Southern train carrying hazardous materials derailed in eastern Ohio and caught fire. The crash was likely caused by an overheating bearing and isn’t one the automatic braking system is designed to prevent.

Chair Jennifer Homendy said Positive Train Control is clearly helping improve rail safety but that “we haven’t achieved zero deaths on our railroads, which means there’s more we can and must do to strengthen safety.”

Railroads have suggested that the braking system could make the second crew member in cab locomotives unneeded, but unions have long opposed the idea of cutting train crews down to one person because of safety concerns.

The National Transportation Safety Board said there are several shortcomings of the current railroad braking system that developed partly because the system had to be designed so that every railroad’s system would work on another railroad. Locomotives are often passed back and forth between railroads to help keep trains moving.

One of the more practical recommendations the board made was for railroads to develop a way to automatically turn the automatic braking system back on after it is manually disabled to allow for common switching movements that involve backing a train onto the main line through a red signal.

Currently, the automatic braking systems will remind an engineer to turn the system back on if they don’t do it within several miles of when the switching move was completed, but the safety board said a derailment can happen before that warning kicks in.

The braking system also uses a combination of a GPS sensor on the locomotive along with trackside sensors to monitor where the engine is and send signals to stop the train if an engineer misses a stop signal or if a crew is working on the tracks ahead.

The National Transportation Safety Board said that too often the system just imposes a low speed limit and relies on engineers to stop their trains in time, which doesn’t always work.

The system also has a hard time preventing rear-end collisions, partly because it isn’t set up to track how long a train is and doesn’t know where the back of a train is. The board recommended that the Federal Railroad Administration and railroads keep looking for a solution to this using a GPS sensor at the end of a train.

The braking system also doesn’t have the ability to stop a train if there is an object like a car or person on the tracks, so the safety board also recommended developing a set of sensors that could help with that.