OMAHA, Neb. — Federal regulators have proposed suspending a Trump administration rule that would have allowed railroads to haul liquefied natural gas while they take a closer look at the potential safety risks.

The rule, which was backed by both the natural gas and freight rail industries, had already been on hold because several environmental groups and 14 states filed lawsuits challenging it.

The federal Pipelines and Hazardous Materials Safety Administration said the uncertainty about the rule also kept companies from investing in the specialized rail tank cars that were required, so railroads haven’t actually handled any shipments of the flammable and odorless liquid known as LNG since the rule was issued last summer.

The rule would have required enhancements — including a thicker outer tank made of steel with a greater puncture resistance — to the approved tank car design that, for decades, has been approved for shipments of other flammable cryogenic materials

But in their lawsuit, environmental groups argued the railcars, which have yet to be built, were untested and might not withstand high-speed impacts.

“We don’t believe that LNG by rail should have ever been authorized in the first place, so we look forward to the authorization being suspended,” said Bradley Marshall, who is a senior attorney with Earthjustice which filed the lawsuit on behalf of the environmental groups.

States that challenged the rule included California, Delaware, Illinois, Maryland, Massachusetts, Michigan, Minnesota, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont and Washington. Washington D.C. was also part of the lawsuit.

The lawsuits have all been put on hold while federal regulators review the rule, which could take until the summer of 2024.

Before the rule was issued last summer, federal hazardous materials regulations allowed shipments of LNG by truck, but not by rail, except with a special permit.

The move to abandon the rule on rail shipments comes as natural gas prices and exports are surging in the United States. And ongoing court and regulatory battles have slowed development of pipelines that would help deliver the nation’s world-leading gas production to markets.

The long-term growth in LNG exports has continued to set records, according to the latest federal figures. U.S. LNG exports averaged 9.6 billion cubic feet per day during the first six months of this year as the economy continued to recover from the coronavirus pandemic.

A spokeswoman for the Association of American Railroads trade group defended the rail industry’s safety track record.

“Railroads continue to be the safest way to move hazardous materials over land with 99.99% of all hazmat shipment arriving at their destination without incident,” said Jessica Kahanek.