The rail industry asked the Department of Transportation three years ago to write new regulations for railroad tank cars that were carrying the country’s nascent oil boom.

In the two years that followed, state and local officials and the National Transportation Safety Board also urged the department to take action.

But the DOT did not begin the rule-making process until last September, two months after 47 people were killed in a violent inferno when a trainload of North Dakota crude oil left the tracks in Quebec and exploded.

The department’s Pipeline and Hazardous Materials Safety Administration is working on the new regulations, and its sister agency, the Federal Railroad Administration, last week announced a series of voluntary measures to improve the safety of crude oil shipments. However, it might be another year before the tank car rules take effect.

It wouldn’t be the first time that rail safety regulations were delayed until tragedy struck. Four decades ago, the DOT required tougher standards for certain types of tank cars carrying flammable gases such as propane. Regulators gave the industry a three-year deadline, but did little to enforce it.

On Feb. 24, 1978, two months after that deadline passed, 16 people were killed in Waverly, Tenn., when a derailed tank car of propane blew up. The accident claimed the lives of the small town’s police chief, fire chief and half its fire department. Many others were badly burned. Only then were the improvements made.

“Washington’s very reactive,” said Mary Schiavo, a DOT inspector general in the Clinton administration. “Most reactive of all is DOT.”

Cynthia Quarterman, the head of the pipeline safety agency, which is writing the new rules on tank cars, probably will face tough questions from lawmakers in a hearing on rail safety oversight Wednesday in the House of Representatives. A key question likely will be why the agency took so long to act.

Jeannie Shiffer, a spokeswoman for the pipeline safety administration, said the DOT was “working aggressively across multiple fronts” on safety issues related to transporting crude oil.

“These are very technical and complex issues that require extensive review,” she said. “As the rule making progresses, if we observe an imminent hazard, we will continue taking action and notifying the public.”

But officials across the country, from mayors to members of Congress, wonder why the agency failed to act with more urgency to protect their communities.

“It’s kind of inexplicable to me why it would take so long,” said Karen Darch, the village president of Barrington, Ill., a Chicago suburb that’s in the path of an increasing volume of crude oil shipments from western Canada to the Gulf Coast.

Darch asked the DOT for new regulations in April 2012.

In a statement, Meghan Keck, a spokeswoman for Transportation Secretary Anthony Foxx, said safety was the department’s top priority “and will continue to be our guiding principle as we work to strengthen safety measures regarding the transportation of crude oil and all forms of transportation.”

The delay could threaten the safety of the communities through which the trains pass, as well as economic growth fueled by the energy boom. States, which have little power to regulate rail transportation, are counting on the federal government.

Railroads aren’t waiting. BNSF Railway is buying 5,000 new tank cars, which is unusual because railroads generally don’t own them. Canada’s two largest railroads will attempt to discourage shippers from using less-safe tank cars by imposing a 5 percent surcharge. At least two refiners are planning to ship crude only in the better cars.

Rail and petroleum industry officials say they want regulatory certainty but won’t openly criticize Quarterman’s agency. However, they’ve been waiting longer than anyone else for an answer.

After it petitioned the agency in March 2011, the rail industry voluntarily adopted tougher safety standards for tank cars later that year. While tank car manufacturers struggled to keep up with the demand for new cars as oil production increased in North Dakota’s Bakken shale region, tens of thousands of older, less-protected tank cars filled the void.

The NTSB had been warning for more than two decades that the general-purpose DOT-111A tank car was vulnerable to punctures and ruptures in derailments involving flammable and toxic materials.

Meanwhile, rail shipments of crude oil from North Dakota continued to grow as production outpaced available pipeline capacity.

“I don’t think in their wildest dreams they thought about a catastrophe,” Darch said.

Then it struck. On July 6, 2013, an unattended 72-car train of Bakken crude oil rolled down a hill and derailed in the lakeside town of Lac-Megantic, Quebec.

On Sept. 6, Quarterman’s agency announced that it would begin the rule-making process. The agency has received tens of thousands of comments from industry and environmental groups, local governments and citizen activists.

On Nov. 8, another Bakken crude oil train derailed near Aliceville, Ala., with a fire as large and intense as the one in Quebec. No one was injured or killed, but the accident spilled 748,000 gallons of crude oil into a wetland.

On Dec. 30, a train derailed near Casselton, N.D. The accident spilled 475,000 gallons of crude oil, some of which burned, sending a column of smoke and fire hundreds of feet into the air. No one was killed, but two-thirds of the nearby town’s residents evacuated.

Federal regulators concluded last month that Bakken crude was more flammable than conventional kinds.

“I think the volume caught DOT off guard,” said Schiavo, the former inspector general. “I don’t think they were on top of what that really meant.”