North Dakota's congressional dele­gation pushed federal railroad regulators Thursday to get moving on regulations to strengthen rail tank cars in light of the fiery explosion of an oil train that derailed Dec. 30 near ­Casselton, N.D.

The meeting with regulators in Washington followed Sen. Heidi Heitkamp's letter Wednesday demanding that the Federal Railroad Administration release inspection reports about tracks near the crash site where four previous derailments had occurred in nine years.

Heitkamp, a Democrat, and Sen. John Hoeven and Rep. Kevin Cramer, both Republicans, met with U.S. Transportation Secretary Anthony Foxx and another regulator working on rail tank car safety.

"We need to upgrade the tanker fleet as aggressively as possible," Hoeven told the Star Tribune after the meeting.

Foxx issued a statement saying that the Department of Transportation would announce "in the coming weeks" other steps to help ensure the safety of hazardous materials on railroads.

Hoeven also released a letter Thursday that he sent to the DOT's Pipeline and Hazardous Materials Safety Administration (PHMSA) more than a year ago urging action on tougher standards for tank cars. He said in an interview that regulators promised to begin proposing regulations in a few weeks.

Regulators are considering whether to order upgrades to older tankers to make them more puncture-resistant and reduce the risk of breaching, catching fire and exploding in accidents.

Up to 65,000 older tank cars that haul oil, ethanol and other flammable liquids potentially could need upgrades — work that rail industry officials say could take a decade. The safety recommendation was made five years ago by the National Transportation Safety Board (NTSB) after a deadly explosion of an ethanol train in Illinois.

3 oil train crashes in 6 months

In the past six months, three oil trains have crashed and exploded, and the worst was in Lac-Mé­gan­tic, Que­bec, where 47 peo­ple were killed July 6.

About 11 percent of the nation's crude oil moves by rail, much of it from North Dakota, which lacks sufficient pipelines to carry its oil bounty.

In a statement, Heitkamp said the delegation "put pressure" on the DOT officials "to step up and move forward with safety provisions, which have not been handled with the necessary urgency."

On Wednesday, Heitkamp also demanded that rail regulators release inspection logs, reports and other evidence of prior track problems around Casselton, a pattern first reported by the Forum newspaper in Fargo.

Federal regulators have said for months that they are testing North Dakota's light crude oil to determine whether it poses a higher hazard than crude oil from other parts of the country because of dissolved, explosive gases or other characteristics. Hoeven said PHMSA Administrator Cynthia Quarterman attended the meeting, but didn't disclose results from the hazard analysis.

In the accident last week outside of Casselton, a BNSF Railway Co. train with more than 100 crude oil tank cars crash­ed into a grain car from an­oth­er train that had de­railed on a par­al­lel track. The re­sul­ting ex­plo­sions, fire and bil­low­ing smoke from 18 tank cars caused a par­tial e­vac­u­a­tion of the near­by town's 2,400 inhabitants, but no one was injured.

BNSF, in a statement Thursday about the prior derailments, said the NTSB investigation of the Casselton crash needs to be completed before the railroad can understand what happened and determine future actions. BNSF said it had invested a record $220 million last year in its North Dakota system.