Photo Courtesy of U.S. Coast Guard

February CSX derailment in West Virginia spilled oil into the Kanawha River. Tracks were inspected three days earlier, the railroad says.

A growing tally of oil-train derailments so far this year in the U.S. and Canada—some involving newer, supposedly safer tank cars—has called more attention to what politicians and environmental groups see as poorly maintained, aging infrastructure throughout North America's freight rail network. To address the public's concern, the American Association of Railroads says its members are spending $29 billion on infrastructure and equipment in 2015 alone.

In a March 9 letter to President Barack Obama, Sen. Tammy Baldwin (D-Wis.) said preliminary reports from the scene of recent oil train accidents suggest "rail infrastructure has been to blame" for the derailments. "We have also seen evidence of deteriorating bridges in our state on rail lines that carry crude oil and have called on railroads to repair them swiftly," she said.

Four trains hauling crude oil have derailed in the U.S. and Canada since mid-February, causing fiery explosions, spilling contents and polluting rivers. On Feb. 14, a 100-car Canadian National Railway train hauling crude oil and diesel fuel derailed in a rural area of Ontario.

Two days later, a 109-car CSX oil train derailed and caught fire near Mount Carbon, W. Va., leaking oil into the Kanawha River. Days earlier, a Burlington Northern-Santa Fe train carrying oil from North Dakota's Bakken shale play broke from the track near Galena, Ill., and a 94-car CN crude oil train derailed near Gogama, Ontario.

Oil train derailments are rising simply because of exploding traffic volumes in the sector, but total freight derailments have fallen year-over-year for decades, says Allan Zarembski, director of the railroad engineering and safety program, University of Delaware. He says creating and maintaining a field of zero risk in the freight rail sector is a political concept, not an engineering one. "Is there any way to get to zero derailments? Is there ever such a thing as zero risk?" Zarembski asks. "For instance, hundreds of trespasser deaths happen every year. Now it's theoretically possible to fence in all 200,000 miles of railroad, but at the cost of $1 million per mile, that's a $200-trillion job."

Zarembski says 33-35% of derailments are caused by the track, but of 200,000 broken rails every year, 199,800 are successfully identified and replaced without a derailment. "Now those numbers can keep improving with improved ultrasonic technologies and inspection scheduling frequency, but there is still a lot of room for innovation," Zarembski says. "There are always ways to reduce risk."

When engineers study the 1,800 train derailments that occur in the U.S. every year, however, they notice numerous categories when it comes to the cause of each specific incident and an "oddball" set of circumstances that cannot always be addressed without a tremendous outlay of resources, Zarembski says. "So do you spend $1 billion to address the cause of a $1-million derailment?" he says. "It's an old question."

But the fact that the railroads are often allowed to answer such questions themselves without government oversight has also been a source of controversy. The Federal Railroad Administration's small number of inspectors—less than 100—has been widely criticized by politicians and environmental groups as oil train incidents mount. Zarembski disagrees with this critique. "It's not necessary for the FRA to replicate the inspection programs of the railroads, which have aggressive inspection programs and thousands of inspectors," he says. "FRA oversees these programs, not the physical railroads themselves."