A week of fiery, crude-filled train derailments has reignited the focus on oil transportation safety. Although the derailments are likely to fuel the ongoing debate about the wisdom of shipping volatile Bakken crude by rail, they are only the latest two flare-ups in what is becoming a long line of explosive incidents.

The derailed trains—one in Ontario on February 14 and another in West Virginia on February 16—were both carrying the especially flammable form of crude along busy rail corridors that spider from Canada to various points in the United States. Although there were no injuries in either accident, the crash outside of Boomer, West Virginia necessitated the evacuation of two towns, spawned a state of emergency declaration, and ignited a fireball that burned for at least three days.

More disturbing than the closely spaced explosions, though, are the rate at which incidences like them have occurred since the Lac-Mégantic tragedy brought the issue to fore in 2013. In the past 12 months there have been at least three other significant derailments in the United States, and a six-fold increase in spills of all kinds from railway tankers, according to the Washington Post.

There are more than a few reasons for that—including the fact that more trains carrying more crude have been traveling the rails since advances in technology allowed oil manufacturers to tap into North Dakota’s Bakken oil formation. That created the need to move crude from the north to refineries across the country. Since 2008, the number of tank cars carrying oil has shot up from about 9,500 to 400,000 according to the Association of American Railroads.

“[The Bakken field operators] were pulling out hundreds of thousands of gallons of barrels a day,” investigative reporter Marcus Stern told NPR. “They were Texas without pipelines. They had all the oil production of Texas…but they didn't have any of the infrastructure in place. And so they very cleverly… turned to the rails because the rails were sitting sort of idle and not used very much except for by grain farmers.”

What ensued was what the Wall Street Journal has termed a virtual pipeline—a steady flow of railway crude that isn’t beholden to the same emergency response planning and inspection requirements that pipelines must follow.

And rather than traversing out-of-the-way locales like a pipeline might, railways drag the erratic material through the high population centers and alongside rivers. That means failures such as the most recent one in Boomer (or the one in Aliceville, Alabama, or the one in Lynchburg, West Virginia) are more likely to have widespread economic and environmental tolls.

It’s not that transportation officials are unaware of the problem. Both Canadian and U.S. officials have been working since Lac-Mégantic to eliminate unsafe practices and create safer tanker standards. The problem though is that efforts thus far have been largely voluntary and emergency orders have been open to railroad company interpretation. The U.S. Transportation Department is currently reviewing proposed rules that would change that.

Until something changes politically, though, the specter of these derailments will loom over every community close to a crude transport line, said Mollie Matteson, senior scientist at the Center for Biological Diversity.

“Back-to-back fiery derailments involving crude oil trains should be an unmistakable wake-up call to our political leaders,” she said in a statement. “People’s lives are at stake, clean drinking water is at stake, and the well-being of towns and wildlife along thousands of miles of rail line are directly in harm’s way of this unchecked, reckless increase in oil transport by rail.”