WASHINGTON -- The explosive derailment of an oil train near Casselton last month has North Dakota's senators pushing harder than ever for expanded oil pipelines.

Both Republican John Hoeven and Democrat Heidi Heitkamp are longtime supporters of expanded pipelines. But the accident in Casselton that produced a fiery wreck and forced residents to briefly evacuate have the senators renewing their pitch to federal officials in recent days.

"I have been working on pipelines for years, both here and in North Dakota," said Hoeven, a former governor. "One thing the [Casselton crash] indicates is that we do need to consider more pipelines."

Hoeven and Heitkamp have met with several federal officials in the aftermath of the Casselton crash, including Transportation Secretary Anthony Foxx. Much of that discussion focused on increased rail safety and pressuring Obama administration officials to speed up their timeline for new rules governing tankers.

But the senators have mentioned pipelines as an alternative or something that needs to be considered in tandem with increased safety procedures. Hoeven said he has brought it up to Foxx and other federal officials. Heitkamp used a meeting with Canada's foreign minister and ambassador to the U.S. earlier this week to press the case for the Keystone XL pipeline, a proposed Canada-to-Texas pipeline that would bring oil from Canada's oil sands and the Bakken Shale in North Dakota to oil refineries near Houston.

Pipeline talk comes as federal investigators continue to look at the Casselton accident. A preliminary federal investigation from the National Transportation Safety Board last week showed that more than 400,000 gallons of oil were lost when a trail derailed and caught fire near Casselton. The Dec. 30 accident also prompted the U.S. Department of Transportation to issue a warning about the volatility of crude from the Bakken oil patch in eastern Montana and western North Dakota.

NTSB's preliminary report said an oil tanker traveling east crashed into a grain car with soybeans that was traveling west. No one was hurt, but the derailments caused a massive explosion just outside of Casselton, and about 1,400 of the town's 2,400 residents voluntarily evacuated. The Casselton derailment came after other recent explosive derailments in Canada and Alabama.

Heitkamp said the derailments showed the need to keep safety in mind as North Dakota's oil industry continues to expand.

While she said it was impossible to avoid transporting oil aboveground and she wouldn't want to build pipelines to the detriment of improving rail safety, Heitkamp also has expressed frustration that the Obama administration has not been more open to oil pipelines, particularly the Keystone XL oil pipeline.

Environmental groups have derided the proposed pipeline as a major contributor to global warming and say they are worried about spills of tar sands oil, which is heavier than conventional oil.

Heitkamp, Hoeven and other backers say the project would create jobs and boost North American energy independence. The project is under review at the State Department. And on Friday, Secretary of State John Kerry said there were still "a lot of questions" about the proposed 1,700-mile pipeline.

"All the appropriate effort is being put into trying to get this done effectively and rapidly," Kerry said.

Heitkamp said pipelines were only one part of the equation, but an important part for North Dakota.

"Even if we had a completely built-out infrastructure we would still have to transport oil by rail at some point," Heitkamp said. "We do need to focus on safety and building out all the infrastructure, and that includes pipelines."

Hoeven said the potential of pipelines was too much to ignore, particularly in a state where much is new. He said just approving Keystone alone would reduce the amount of oil transported through towns and cities.

"With the fast growth we've had some challenges, and I think we need the infrastructure to move that growth to market and of course we need to do that all as safely as possible," Hoeven said. "That includes pipelines. It has to include them."