A pair of senior Democratic senators demanded Thursday that the Obama administration investigate the safety of oil tanker trains following two recent accidents.

“The recent derailments and accidents involving crude oil are alarming and demand increased vigilance. We urge you to work together to quickly resolve issues with the Transportation of crude oil in order to protect our communities, and prevent any further disasters,” Senate Commerce, Science and Transportation Chairman Jay Rockefeller, D-W.Va., and Energy and Natural Resources Chairman Ron Wyden, D-Ore., wrote to Energy Secretary Ernest Moniz and Transportation Secretary Anthony Foxx.

The letter comes after a train carrying crude crashed Dec. 31, causing 21 railcars to catch fire in Casselton, N.D., and forcing 2,000 people from their homes to escape the fumes. And on Wednesday, another tanker went ablaze after it derailed in New Brunswick, Canada. No one was hurt in either incident.

Crude transport by rail has grown alongside rapid oil and gas production, as the nation's pipeline infrastructure has failed to keep pace with rising energy production spurred by hydraulic fracturing, or fracking. In their letter, Wyden and Rockefeller noted that the North Dakota Pipeline Authority projected railroad oil shipping capacity from the state would top 2.5 million barrels per day in 2016, which they said nearly doubled pipeline capacity.

Specifically, the senators want federal officials to get a better gauge on the amount of crude being transported by rail, improve classification of different types of crude, and enhance safety regulations.

Some of that is already underway.

The Transportation Department's Pipeline and Hazardous Materials Safety Administration began the process to strengthen regulations for the tankers that carry crude following a July oil tanker explosion in Quebec that killed 47 people. Some of the changes could include "enhanced tank head and shell puncture resistance systems for ... tank cars, as well as top fittings protection that exceed current requirements," the agency said.

And the safety administration said in a safety notice last week after the Casselton accident that the oil coming from the heart of the energy boom -- the Bakken shale formation in North Dakota and Montana -- might be more flammable than traditional crude. As a result, the agency is expanding the scope of an ongoing effort to properly classify crude.

The incidents have caused alarm on Capitol Hill. North Dakota Sens. John Hoeven, a Republican, and Heidi Heitkamp, a Democrat, met with Foxx and PHMSA chief Cynthia Quarterman on Thursday to discuss railway safety.

"Everyone has an interest in railway safety — federal regulators, railroads, shippers and certainly communities along the tracks,” Hoeven said. “Consequently, we need everyone to be involved to address it."

The oil and gas industry maintains that transporting crude via tanker is safe, and the American Association of Railroads said 99.9973 percent of hazardous material shipments reach their destination safely.

"Flammability, classification, issues like that are very important. We should talk about those. But we shouldn't ever forget the fundamental root cause that's taking place to cause the incident. If it's because the rail car has gone off of the track, we should make sure the rail cars don't go off the track," American Petroleum Institute CEO Jack Gerard said Tuesday at his organization's annual State of American Energy speech.