A timber rail bridge burned and collapsed early Wednesday in Koochiching County, tipping two rail cars onto the banks of the Rat Root River and temporarily blocking a Canadian National rail artery that connects the Pacific Coast with Chicago.

The cause of the fire is still unclear, though the state fire marshal is helping with the investigation, said Perryn Hedlund, sheriff of Koochiching County.

"We're looking at every angle; we're not jumping to conclusions," Hedlund said.

The bridge collapse blocks an increasingly busy Canadian National line that runs from Prince Rupert, British Columbia, to Chicago. The route has made Ranier, Minn., just east of International Falls, one of the busiest rail crossings on the U.S.-Canada border.

About 12:35 a.m., the crew of a southbound Canadian National train reported a fire on the trestle as they approached the bridge just north of Ericsburg, 10 miles south of International Falls.

"When he came around the bend, he saw that the bridge was on fire. He hit the emergency brake, but obviously he couldn't stop in time," Hedlund said. "About 20 cars had passed the bridge, and then he of course comes to a stop on a burning bridge, which creates a whole new issue."

Graphic: Map: Bridge collapse near International Falls Graphic: Map: Bridge collapse near International Falls

The bridge burned for a couple of hours before it collapsed, Hedlund said. Because of the darkness, the terrain and the fact the cars contained potash, a fertilizer, the fire department was reluctant to send firefighters too close to the fire.

"It's not worth someone losing their life over," Hedlund said.

No one was injured, and workers were pumping potash out of the cars hanging in the wreckage as Hedlund left the scene Wednesday afternoon.

All rail traffic through Ranier is now halted, but the line should be up and running shortly, said Larry Gross, a rail expert for FTR Transportation Intelligence.

"It won't take them long," Gross said. "I'd say they'll have it back within a day or two."

A spokesman for CN did not respond to questions about how long it will take to rebuild the bridge.

The railroads generally help each other in situations like this, Gross said, and CN could reroute eastbound trains onto BNSF and Canadian Pacific lines that come down from Manitoba into northwest Minnesota.

The incident highlights growing anxiety in Ranier, a village of 150 that has seen freight rail traffic from Canada surge in recent years.

The bridge across the Rainy River from Fort Frances, Ontario, to Ranier was built in 1907 and had long been a quiet crossing. But now it carries more than 20 trains a day that are up to two miles long, said Dennis Wagner, mayor of Ranier.

"Something's going to happen," Wagner said. "It's just freaking math."

Wagner said about 19 percent of all freight rail traffic across the border passes through Ranier and then Ericsburg. The burned bridge makes him worry that a similar incident could happen in downtown Ranier.

"What happens when this other bridge that's 120 years old collapses? Oh! Imagine that. And then it fills the whole Rainy River full of oil and gas," he said. "Rail safety and bridge safety has been an issue of major concern around here."