“These new type of cars were supposed to be safer, but it’s obvious these cars are not good enough or safe enough,” said Claude Gravelle, a Canadian lawmaker who represents the northern Ontario area where two recent derailments occurred.

On Sunday, emergency workers were still trying to extinguish fires in multiple tank cars after 30 cars of a 94-car Canadian National Railway Co. train laden with Alberta crude derailed Saturday near Gogoma, Ontario. Five cars landed in a waterway.

The energy industry began using rail to transport oil in 2008 because it was a fast and inexpensive way to move growing volumes largely from the Bakken Shale in North Dakota.

In addition, building new pipelines has been expensive and politically fraught. In February, President Barack Obama vetoed legislation to approve the Keystone XL pipeline, which has been under review by the Obama administration for more than six years.

The robustness of tanker cars has become a major focus of efforts to improve the safety of shipping crude by rail. Such shipments have soared from about 21,200 barrels a day in 2009 to 1.04 million barrels a day by the end of 2014, according to government statistics.

As the U.S. shale boom gathered speed, the safety of growing crude shipments by rail has attracted greater scrutiny in the U.S. and Canada, especially after a 2013 derailment in Lac-Mégantic, Quebec, that claimed 47 lives.

Speed limits have been adopted, and a new rule in North Dakota that will take effect next month requires crude from the state to be treated to make the crude less combustible.

The cars involved in the two Ontario derailments and the incidents in West Virginia and Illinois all met the standards introduced by the rail industry in 2011 as a significant upgrade over older models, and were built with thicker shells and pressure-relief devices.

There are about 60,000 of the new CPC-1232 tanker cars in use hauling crude oil across North America, as well as about 100,000 of the older models, says the Association of American Railroads.

Last year, the Transportation Department proposed additional new rules for tank cars carrying crude, presenting three main options. One would stick with the CPC-1232, but the other two would make new cars stronger and retrofit existing cars.

The White House is now reviewing these options and is expected to issue recommendations in May.

Ed Greenberg, a spokesman for the Association of American Railroads, said the railroad-industry trade group “wants all tank cars carrying crude oil, including the CPC-1232, to be upgraded by retrofitting or taken out of service. Railroads share the public’s deep concern regarding the safe movement of crude oil by rail.”

The American Petroleum Institute, the oil industry’s trade group, says it also supports upgrades to the tanker fleet to improve safety.

Cynthia Quarterman, a former director of the Pipeline and Hazardous Materials Safety Administration who stepped down last October, said the recent incidents “confirm that the CPC-1232 just doesn’t cut it.”

Tanker-car improvements alone won’t be enough to reduce overall risk, she added. “The crashworthiness of the tank cars does need to be raised, but that’s not enough. There needs to be a comprehensive solution, including better brakes to help minimize pileups.”

The four recent crashes also highlight some of the other risks of carrying crude by rail that seem to be persistent.

Two of the derailments involved Bakken crude from North Dakota, which contains a high level of gas, making it more volatile than other kinds of crude. In the Mount Carbon, W.Va., accident in February, nearly two dozen tankers full of crude derailed and were engulfed in flames, some exploding into fireballs that rose more than 100 feet in the air.

Tests on the crude showed that its vapor pressure, a measure of volatility, exceeded a new regulatory standard that will go into effect next month.

The recent derailments involved long trains that are essentially mobile pipelines as much as a mile long. The BNSF Railway Co. train that derailed and caught fire in Galena, Ill., 160 miles northwest of Chicago, was roughly a mile long and carrying 103 railcars loaded with crude from North Dakota’s Bakken Shale. BNSF is a unit of  Berkshire Hathaway Inc.

“We certainly believe that a stronger tank car is necessary and appropriate,” said Mike Treviño, a BNSF spokesman. A Canadian National spokesman said the company is in favor of stronger tank-car design standards.

The train in the Canadian National accident in Ontario over the weekend was 94 cars long, while the West Virginia train had 109 tankers full of North Dakota crude oil.

Canadian Transport Minister Lisa Raitt referred to “very long” unit trains last month when she proposed a new tax on crude shipments by rail aimed at building an insurance fund. “With that increased length of car, there’s an increased risk associated with it,” she said.

The number of derailments on long-haul tracks in the U.S. has declined 21% since 2009, according to the Federal Railroad Administration. But the number of train accidents related to “fire” or “violent rupture” climbed to 38 last year from 20 in 2009.