The 106-page report from the NAS released on September 17 details how longer trains have created "new and heightened safety risks," which have made it harder to maneuver rail cars around curves and avoid a potential derailment. Those difficulties are magnified when trains have cars that vary in size, shape and weight, particularly when those rail cars are poorly positioned.

"As the length of a manifest train increases, so too will the complexity of accounting for these in-train forces through train makeup decisions," the report reads. 

The NAS estimates that the average train length increased by roughly 25% between 2008 and 2017, and that by 2021, around a quarter of all trains were more than a mile-and-a-half long, with some measuring upwards of two-and-a-half miles. In some instances, trains have gotten so long that crew members situated at the front have found that their radios will often not be able to reach workers in the rear rail cars. 

In the case of a fiery derailment in Bedford County, Pennsylvania in 2017, the National Transportation Safety Board (NTSB) found that the 178-car train was more than two miles long. After the derailment, the NTSB raised questions about the railroad's decision to position empty rail cars at the front of the train, describing how the heavier, loaded cars that came off the tracks had pushed on the train as it descended a grade. 

The report calls on Congress to empower the Federal Railroad Administration to address the risks posed by longer trains, and recommends requiring railways to carefully plan the makeup of trains to account for those dangers. It also says that railroads should be penalized for violating those proposed requirements.