Investigators haven’t confirmed the cause of the Sept. 14 blast in a remote corner of the railroad’s Bailey Yard in North Platte, Nebraska, about 250 miles west of Omaha. The explosion didn’t spread far, but investigators appear to be delving into the questionable decision to load dozens of plastic barrels of perchloric acid inside a shipping container with a wood floor and possibly atop wooden pallets, even though that acid is known to react with wood or any other organic material.

“I don’t know if you’ve ever read about perchloric acid, but when it comes in contact with organic material, it becomes highly volatile. So that car was doomed from the day it was loaded,” said Andy Foust, a Nebraska leader of the largest rail union that represents the workers who were switching those railcars just before the explosion.

The explosion highlighted not only potential problems at the sprawling railyard but also the national rail network’s reliance on everyone involved in shipping hazardous materials taking proper precautions. As the Nebraska explosion made clear, there can be problems that are hard to spot before potentially disastrous accidents occur.

Some details about the explosion might never be known because the shipping container carrying the acid was destroyed. Federal Railroad Administration spokesman Warren Flatau said “the leaked acid reacted with the wooden floor of the intermodal container, and any other organic material within the container (i.e., pallets).”

The resulting explosion propelled shrapnel up to 600 feet away and prompted first responders to evacuate everyone within a mile outside the railyard. After the first container exploded, a second metal shipping container — believed to hold memory foam — fell down on top of it and caught fire, but no other cars ignited.

U.S. Transportation Secretary Pete Buttigieg told a gathering of rail labor leaders in Nebraska the explosion could have been much worse and “grabbed national headlines had the wind been blowing a little bit different or had things gone just a little bit different in the yard that day” — much like Norfolk Southern’s fiery derailment in eastern Ohio did.

That February derailment — and others that followed — put the focus on railroad safety and prompted Congress and regulators to propose reforms, which have largely stalled.

Foust said Union Pacific never evacuated the railyard. Nearby workers left the area on their own, but most in the railyard continued working. Foust expects that to change because he said UP is revising its emergency response plan.

“There was a large part of that yard that had no idea what was going on, and they were told to continue doing their job,” said Foust, who has discussed the explosion with FRA inspectors, the railroad and first responders because of his role with the Sheet Metal, Air, Rail and Transportation Workers-Transportation Division union.

Railroad officials said at the time that the wind blew smoke away from the facility, and because the railyard is up to eight miles (13 kilometers) wide, most UP workers were a safe distance away.

Despite the explosion, the method of loading 56 barrels of acid doesn’t appear to violate any regulations, so such shipping methods could still be occurring. A Pipelines and Hazardous Materials Safety Administration spokesman said that “as the regulations prohibit any leakage from a package, the regulations do not specify what materials the drums are loaded onto.”

Shippers are required to take precautions when loading hazardous materials, including ensuring that plastic drums can’t tip over. The drums used to ship hazardous materials also must undergo extensive testing.

In this case, the FRA spokesman said investigators couldn’t determine what loading precautions were taken because the container was destroyed.

The perchloric acid, used in explosives and some industrial processes, was produced at a company in Ohio that hasn’t been publicly identified. Norfolk Southern transported the acid, then handed it off to Union Pacific. Both railroads declined to comment on the explosion, citing the ongoing investigation.

Given the timing of the explosion, the leak likely happened inside UP’s railyard.

Railroads inspect railcars before they pick them up for mechanical problems or signs of tampering, but they assume shippers have properly packed them.

“We really rely on them to know best how to package and do it in a safe way, so it isn’t going to become an issue for us,” said North Platte Fire Chief Dennis Thompson, who led the response to the explosion.

Thompson said the emergency response went smoothly because the weather conditions and location were favorable, and the railroad let him know within 25 minutes exactly what they were dealing with.

Before the explosion, inspections at the railyard in July and August prompted the head of the FRA to write a letter to UP’s CEO highlighting that the rate of defects was twice the national average. In September, the week after the explosion, inspectors returned to follow up and turned up more than 500 additional problems.

Union Pacific CEO Jim Vena said he understands and welcomes the agency’s scrutiny.

Vena said Union Pacific and other major railroads have become safer over time. But there were still more than 1,000 derailments last year, and as the East Palestine derailment demonstrated, just one train crash can be disastrous if hazardous materials are involved.

“Do we have more to do? Absolutely,” said Vena, who became CEO in August. “And that’s what I’m challenging the team with here at Union Pacific is we have to get better … We’ll invest in it. We’ll spend money on technology. We’ll spend money on people.”

FRA regulators who oversee the inspectors aren’t overly concerned, with the head of the agency’s Office of Railroad Infrastructure and Mechanical saying violations are common when he sends a team out.

“We did not find any systematic issues that would indicate they are operating unsafe equipment that put the public at risk,” the FRA’s Charlie King said.