WASHINGTON

The Obama administration has delayed by nearly a year a plan to boost safety standards for the type of rail car involved in a fiery explosion that killed at least 47 people in Canada this month.

Officials began work on the rule more than a year before an oil train derailed and exploded in Quebec on July 6 — but the rule was never put in place. The proposal by the Pipeline and Hazardous Materials Safety Administration is intended to fix a dangerous design flaw in a rail car commonly used to haul oil and other hazardous liquids from coast to coast. The soda-can shaped car, known as the DOT-111, has come under scrutiny from safety experts because of its tendency to split open during derailments and other major accidents.

That’s exactly what happened when an unattended Montreal, Maine & Atlantic Railway train came loose, hurtling down a 7-mile incline before derailing and igniting in Lac-Megantic, near the Maine border. All but one of its 73 cars were carrying crude oil, and at least five exploded, setting off massive explosions that devastated the small lakeside town of 6,000 people.

The structure of the tank car is not believed to be a factor in the derailment, which is under investigation. But transportation experts say the car’s underlying design makes it prone to damage and catastrophic loss of hazardous materials.

A proposed rule to beef up rail-car safety was initially scheduled to be put in place last October, but it has been delayed until late September at the earliest. A final rule isn’t expected until next year.

The pipeline safety agency said in a report this month that the latest delay was needed to allow “additional coordination” among officials and interested groups, including industry representatives who have resisted calls to retrofit existing cars, citing the expense and technical challenges such a requirement would pose.

In the first half of this year, U.S. railroads moved 178,000 carloads of crude oil. That’s double the number during the same period last year and 33 times more than during the same period in 2009. The Railway Association of Canada estimates that as many as 140,000 carloads of crude oil will be shipped on Canada’s tracks this year, up from 500 carloads in 2009. Much of that increase is from oil produced in the Bakken oil patch in North Dakota and Montana and surrounding areas. The train that crashed in Quebec was carrying oil from North Dakota to a refinery in New Brunswick, Canada.

The DOT-111 tank car represents more than two-thirds of the rail fleet carrying crude oil.

The Associated Press reported in September that the DOT-111 tank car has been allowed to haul hazardous liquids from coast to coast even though transportation officials were aware of the design flaw.

The AP had reviewed 20 years of federal rail accident data involving DOT-111 cars used to haul ethanol and found that the cars had been breached in at least 40 serious accidents since 2000. In the previous decade, there were just two breaches.

Sen. Charles Schumer, DN. Y., is urging the Obama administration to phase-out DOT-111 tank cars or require freight rail carriers to retrofit them to prevent potential explosions or spills. Schumer described the DOT-111 cars as “flawed, out-of-date and a factor in hazardous material spills during derailments,” such as the one in Quebec.

While freight rail should not be “demonized,” increased traffic of rail cars carrying crude oil “warrants increased safety measures, and that begins with putting the safest, most up-to-date tank cars on the tracks,” Schumer said at a news conference last week in Albany, N.Y.

Democratic Reps. Mike Michaud and Chellie Pingree of Maine said the government should conduct a wideranging safety review as transportation of oil by train increases dramatically in Maine and other states. Nearly 30,000 barrels of oil per day crossed Maine in March — 15 times more than the same period a year earlier, the lawmakers said in a letter to National Transportation Safety Board and other officials. The train that derailed in Quebec was scheduled to cross Maine on its way to a refinery in St. John, New Brunswick.

A spokeswoman for the American Association of Railroads, which represents the rail industry, said the group shares Schumer’s belief in putting safety first.

“If safer and better DOT- 111s can be had, then it makes good sense to ensure that the design and standards that these cars are built to, must be tougher than the federal standards that exist today,” said spokeswoman Patricia Reilly.

Rather than waiting for the Obama administration to act, Reilly said, the industry has adopted voluntary standards ensuring that all DOT-111s ordered after October 2011 meet tough requirements recommended by the NTSB after a deadly ethanol train derailment and explosion in Illinois in 2009.

But those voluntary standards do not apply to an estimated 40,000 cars built before October 2011 that carry oil, ethanol and other flammable liquids.

The railroads and the oil industry have resisted calls to retrofit existing cars, saying that would present technological and engineering challenges and cost at least $1 billion.

The DOT-111 car’s steel shell is too thin to resist puncture in accidents, the NTSB said, and the ends of the car are vulnerable to ruptures. Valves used for unloading and other exposed fittings on the tops of the tankers can also break during rollovers.