BILLINGS, MONT. – BNSF Railway Co. said Thursday it intends to buy a fleet of 5,000 strengthened tank cars to haul oil and ethanol in a move that would set a higher safety standard for a fleet that's seen multiple major accidents.

The voluntary step by the Texas-based subsidiary of Warren Buffett's Berkshire Hathaway Inc. comes as railroads in the United States and Canada are under intense pressure to improve safety for hazardous materials shipments.

There's been a string of recent train accidents involving oil and ethanol, punctuated by a shipment of North Dakota crude oil that derailed in Quebec last July and killed 47 people.

A boom in domestic oil drilling and rising ethanol production has spurred a dramatic increase in shipments of the materials by rail. Much of it is being hauled by an old fleet of 78,000 tank cars that are prone to split during accidents.

Thursday's announcement marks a potential major step in addressing that problem. However, it would not mean those older cars would go away, and there's already a two-year backlog on new tank-car construction.

In announcing that it will ask manufacturers for bids on the new cars, BNSF indicated that it is unwilling to wait for the U.S. Department of Transportation to finalize regulations on improved tank cars.

The company said it hoped to accelerate the transition to a new generation of safer tank cars and give manufacturers a head start in designing them as federal officials consider changes to current standards.

Typically, railroads don't own the tank cars they pull. But whether BNSF's proposal will spur other shipping companies or railroads to follow suit was uncertain, said Tom Simpson, president of the Railway Supply Institute, a trade association representing tank car manufacturers and owners.

"Everyone has the right to go to a tougher standard," Simpson said. "We'll see how it plays out.

BNSF spokeswoman Roxanne Butler said the request for bids on new cars reflects the company's "commitment to crude-by-rail growth and improving the overall safety of crude transportation."

Among the companies that will get a request for proposals is Union Tank Car, which also is owned by Berkshire Hathaway, Butler said.

Bloomberg News contributed to this report.