BILLINGS, Mont. — U.S. safety officials said Tuesday they’ve seen slow progress in efforts to upgrade or replace tens of thousands of rupture-prone rail cars used to transport oil and ethanol, despite numerous fiery derailments that prompted new rules for the industry.

Figures provided by the Association of American Railroads indicate just over 10,000 stronger tank cars mandated by the new rules are available for service. That’s equivalent to roughly one-fifth of the 51,500 tank cars used to haul crude and ethanol during the first quarter of 2016.

Fiery accidents and spills involving the older tank cars have occurred in Oregon, Montana, North Dakota, Illinois, West Virginia and Quebec, where 47 people were killed when a runaway oil train derailed in 2013.

The most recent accident occurred last month in Oregon, where 42,000 gallons of crude oil spilled, sparking a massive fire.

National Transportation Safety Board member Robert Sumwalt told The Associated Press that federal regulators need to set milestones to hold the industry accountable for getting unsafe cars off the tracks.

“There’s been 28 accidents over the past 10 years. That’s almost three accidents a year,” Sumwalt said. “Unfortunately, history shows we probably will have more accidents involving flammable liquids.”

Sumwalt spoke ahead of a planned meeting in Washington, D.C., on Wednesday, where government and industry officials were to update the safety board on progress addressing the issue.