BILLINGS, Mont. (AP) — U.S. safety officials say 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 a string of fiery derailments.

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

Figures provided by the Association of American Railroads indicate just over 10,000 stronger tank cars 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. The NTSB says all of the older cars are prone to rupture.

Tom Simpson with the Railway Supply Institute says the industry is committed to putting stronger cars in place but demand has eased as shipments have decreased with lower oil prices.