The glut of new oil in North America has been accompanied by a boom in moving that petroleum by train. Railway traffic of crude oil in tankers has more than doubled in volume since 2011—and such transport led to tragedy in the early hours of July 6. At least 13 people were killed in the town of Lac-Megantic, Quebec, when 72 runaway railcars carrying crude oil derailed, crashed and exploded.

Compare that with the leak of diluted bitumen from Alberta's tar sands in Mayflower, Ark., which left a mess but killed no one. The accidents highlight the differences between transporting oil by rail and pipeline. Train transport spills far fewer barrels of oil, but pipeline accidents tend to be more benign, if also more common.

As the Association of American Railroads points out, the volume of oil spilled by railcars is "less than 1 percent of the total pipeline spills." That's 2,268 barrels spilled by the railroads between 2002 and 2012, compared to 474,441 barrels spilled by pipeline operators over the same span, according to the Association's numbers. But there hasn't been a fatality from a petroleum-related pipeline accident since 1999, when a gasoline pipeline ruptured and the volatile fuel exploded, killing three young boys. Deadly accidents involving crude pipelines are even rarer. "Crude oil pipeline spills generally do not result in a fire or an explosion," says attorney Brigham McCown, the first administrator of the Pipeline and Hazardous Materials Safety Administration. "Pipelines transport the lion's share of crude oil because they are the safest, most environmentally friendly and least expensive way to transport large volumes of energy products."

But the boom in unconventional oil, from the tar sands of Alberta to the oil shale of North Dakota, has outpaced the growth of the pipeline infrastructure that would move those products. Efforts to build more pipelines, such as the Keystone XL project that would move oil from the tar sands to Texas's Gulf Coast, have faced significant political and environmental challenges. And, regardless of whether new pipelines are built, the glut of oil from new sources suggests that a variety of transport methods will be needed, in what might be dubbed an "all of the above" approach to energy transportation.

Ultimately, the problem is the unflagging demand for oil, which ceaselessly adds to the greenhouse gases accumulating in the atmosphere and changing the global climate and necessitates moving large quantities of crude around North America, opening up the potential for fatal accidents and spills. Until the U.S. and the world significantly cut oil use, petroleum will continue to move by train, truck, pipeline and supertanker, each with its own risks.