The recent spate of accidents in the U.S. and Canada involving trains carrying crude oil demonstrates that “far too often, safety has been compromised,” the head of the top U.S. transportation safety agency said Tuesday.

The amount of crude oil transported on railroads has more than quadrupled since 2005, and some of it is especially volatile, said National Transportation Safety Board (NTSB) Chairwoman Deborah Hersman.

That extra volatility increases the likelihood of a violent fire in a derailment, Hersman said. The transport of ethanol, the most frequently shipped hazardous material in the railway system, has also boomed, she said.

“With so much flammable liquid carried by rail, it is incumbent upon the rail industry, shippers and regulators to ensure that these hazardous materials are being moved safely,” Hersman said.

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She spoke in Washington, D.C., at the opening of a two-day forum on improving the safety of crude oil and ethanol shipments. The NTSB says that older models of the type of tank car used to transport crude oil and ethanol, known as the DOT-111, are not safe to carry hazardous liquids.

Hersman cited the loss of lives and the destruction that occurred after fiery derailments such as the one on July 6, 2013, in Lac-Mégantic, Quebec; and on June 19, 2009, in Cherry Valley, Ill., near Rockford.

In the Cherry Valley incident, 15 DOT-111s carrying ethanol derailed. The leaking fuel ignited, causing a massive fireball.

One woman was fatally burned and 600 homes within a half-mile radius were evacuated.

The NTSB concluded in a report on that derailment that the design of the older DOT-111 cars made them “susceptible to damage and catastrophic loss of hazardous material.”

Testifying Tuesday at the NTSB forum were representatives from the petroleum and rail industries, who discussed tank-car design, crash worthiness and railroad operations.

Also testifying were researchers who reviewed safety systems and ways to reduce risks.

The NTSB recommended in 2009 that all new and existing tank cars in crude oil and ethanol service be equipped with additional safety design features, including enhanced puncture resistance system, top fittings protection and bottom outlet valves that remain closed during accidents.

Two federal agencies, the Pipeline and Hazardous Materials Safety Administration and the Federal Railroad Administration, are developing a proposed rule to update the federal design standards for DOT-111 tank cars.

Although the petroleum and rail industry and federal regulators agree that new standards are needed, they are divided over whether the requirements should be more stringent than a voluntary industry standard adopted in 2011.

Karen Darch, the village president of Barrington, Ill., and co-chairwoman of a coalition of suburbs calling for tighter standards on the DOT-111s, said Tuesday she is pleased the NTSB is focusing attention on the problem.

Darch and Tom Weisner, mayor of Aurora, Ill., have expressed alarm as ever-increasing crude-oil shipments move through their communities.

But the government’s ongoing testing, quest for consensus among industry and regulators and failure to issue a new tank car rule is “hampering progress toward greater safety,” Darch said.

“Industry does not want to invest in new tank cars until (the U.S. Department of Transportation) has the standard out,” Darch said. “It cannot come quickly enough for us.”

In the Pacific Northwest, about 17 million gallons of petroleum already works its way by rail to refineries in Anacortes, Ferndale in Whatcom County, and Clatskanie, Ore., near the mouth of the Columbia.

That number is expected to more than triple by the end of 2014.