OMAHA, Neb. (AP) — As freight trains have grown ever longer, the number of derailments related to the forces created when railcars push and pull against each other also increased, so the National Academies of Sciences said Tuesday in a long-awaited report that regulators, Congress and the industry should reexamine the risks associated with them.

The report said there is a clear correlation between the number of derailments related to in-train forces and the long trains that routinely measure more than a mile or two long. So railroads must take special care in the way they assemble long trains, especially those with a mix of different types of cars.

That recommendation echoes a warning the Federal Railroad Administration issued last year.

“Long trains aren’t inherently dangerous. But if you don’t have adequate planning on how to put the train together, they can be,” said Peter Swan, a Penn State University professor who was one of the report’s authors.

The increased use of long trains has allowed the major freight railroads — CSX, Union Pacific, BNSF, Norfolk Southern, CPKC and Canadian National — to cut costs because they can employ fewer crews and maintain fewer locomotives. The average length of trains increased by about 25% from 2008 to 2017. By 2021, when the report was commissioned, some trains had grown to nearly 14,000 feet (4,267 meters), or more than 2 1/2 miles (4 kilometers) long.

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The unions representing train crews have said that longer trains are harder to handle, especially when they travel across uneven territory, because of the way cars push and pull against each other. On a train that’s more than a mile long, one section can be going uphill while another section is going downhill. And these trains are so long that the radios rail workers use might not work over the entire distance.

“Anybody and everybody that’s in rail safety knows that this is a problem. It cannot be overstated,” said Jared Cassity, the top safety expert at the SMART-TD union that represents conductors. “Long trains absolutely are a risk to the public and a risk to the workers and anybody with common sense can see that.”

Mark Wallace with the Brotherhood of Locomotive Engineers and Trainmen said Tuesday’s report reinforces what engineers have long known: “Long trains have a greater risk of derailing, have communications issues, and pose a threat to the public due to blocked crossings, among other issues.” The union urged Congress and regulators to act quickly address those risks.

The railroads maintain that their trains are safe at any length. The president and CEO of the Association of American Railroads trade group, Ian Jefferies, said safety is a top priority and many railroads use software that helps them model train forces before railcars are hooked together.

“As operations continue to evolve, railroads are pulling on three key levers — technology, training and infrastructure — to further enhance safety and reliability,” Jefferies said.

But Cassity said countless derailments over the years have shown that train builder software and the cruise control systems that help engineers operate a train are imperfect.

The number of derailments in the U.S. has held steady at more than 1,000 a year, or more than three a day, even as rail traffic decreased. That has gotten attention since the disastrous Norfolk Southern derailment in East Palestine, Ohio, in February 2023 in which hazardous chemicals leaked and burned for days. That train had more than 149 cars and was well over a mile long. The National Transportation Safety Board determined that derailment was caused by an overheating bearing that wasn’t caught in time by trackside sensors.

With long trains, the biggest concern is related to derailments caused by the forces that can tear a train apart as it crosses the countryside. The new report said Congress should make sure the FRA has the power to address the dangers of those trains, and that agency should require railroads to plan carefully on how they handle longer trains.

Railroads can make long trains easier to control by including locomotives in the middle and back of them to help pull and stop them, which is common.

The report said it’s also important for railroads to take great care in where they place heavy tank cars and empty cars and specialized cars equipped with shock absorbers.

In addition to the derailment concerns, long trains can block crossings for extended periods, sometimes cutting off ambulance and police access to entire sections of their communities. They also cause delays for Amtrak passenger trains that get stuck behind monster freight trains that can’t fit within side tracks that are supposed to allow trains to pass each other in such situations.

The report said Congress should give federal regulators the power to penalize railroads for causing such problems.