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A Union Pacific railroad train derailed on a St. Paul bridge connecting two rail yards, puncturing a tanker and causing 3,200 gallons of diesel fuel to dump into the Mississippi River early Wednesday.

The railroad company put four booms into the water to try to contain the spill, including one as far as 8 miles downriver, according to spokeswoman Kristen South. Although fuel was detected at that point, none has been found beyond it, she said. Specially equipped boats were out trying to suck up the fuel. There were no injuries.

"There is no immediate danger to people that we are aware of at this time. Reconnaissance crews are on the river looking for diesel oil contamination," the U.S. Environmental Protection Agency said in a statement.

The U.S. Coast Guard closed the river at the bridge, according to a spokesman, but reports said it remained open for some barge traffic. The spill occurred at mile 835.7, which is near Pig's Eye Lake in Ramsey County.

About 2 a.m., two trains on the bridge derailed, with wheels dislodging from the track, but they did not tip over, South said. The company hasn't determined the cause of the derailment, and the bridge is closed pending inspection.

Minnesota Pollution Control Agency (MPCA) spokesman Walker Smith said the spill was spread out and didn't appear to pose a danger to wildlife or humans, nor was there a threat of fire. Minneapolis and St. Paul get their water farther upstream from the spill.

Smith said Union Pacific will monitor the riverbanks in coming weeks for contamination. Nonetheless, he said, "it's probably best if people don't boat around there."

Significant fuel spills into the river are highly unusual. To compare, Smith said last fall a tractor-trailer spilled up to 80 gallons of fuel on a nearby roadway that made it into storm sewers that empty into the river. "This is certainly the biggest [spill] we've seen on the river in quite a while," he said.

Initial indications are the incident was purely accidental, he said. "Our primary concern is the party responsible reports it right away and they did," Smith said.

The Hoffman Bridge, which opened in 1910, is also called the Chicago & North Western Bridge. The swing bridge is 1,275 feet long and sits 20 feet above the river. The bridge is mostly hidden by trees, but it is near the St. Paul Downtown Airport.

South said the railcars usually haul commodities between the rail yards connected by the bridge, including grain, sugar beets, sand and finished vehicles.

Rochelle Olson • 612-673-1747 Twitter: @rochelleolson