A highway overpass in southeast Missouri collapsed early Saturday when rail cars slammed into one of the bridge's pillars after a cargo train collision, authorities said. Seven people were injured, though none seriously.

The bridge collapsed after a Union Pacific train hit the side of a Burlington Northern Santa Fe train at a rail intersection. Derailed rail cars then hit columns supporting the Highway M overpass, causing it to buckle and partially collapse.

The National Transportation Safety Board launched an investigation into the cause of the cargo train collision, which happened about 2:30 a.m. near Chaffee, a town of about 3,000 southwest of Cape Girardeau.

Only two vehicles were on the overpass at the time. Five people in the vehicles were taken to Saint Francis Medical Center in Cape Girardeau, as were a Union Pacific train conductor and an engineer. All seven had been released by Saturday afternoon, hospital spokeswoman Felecia Blanton said.

"You're driving down the road and the next thing you know the bridge is not there. ... It could have been really bad," Scott County Sheriff Rick Walter said.

The crash derailed about two dozen rail cars hauling scrap metal, automobiles and auto parts, tossing them into the overpass' support columns. The highway was shut down for about 8 miles from Scott City to Chaffee.

The overpass was about 15 years old and in good condition but just couldn't withstand the impact from the rail cars, Walter said.

Two 40-foot sections of the overpass buckled while two cars were on the roadway, sending the cars into the edges of the collapsed sections. A diesel fire also broke out in one of the locomotives after the collision, but was quickly extinguished, Walter said.

When Blanton heard about the crash, she immediately went online and saw video footage of the scene and was bracing for the worst, Blanton said. She said it was "a real blessing" that the injuries were relatively minor, the most serious being a fracture.

"If you look at the pictures, they're very dramatic, and there are no serious injuries," she said. "So it's amazing."

Walter said Deputy Justin Wooten was among the first at the scene and pulled the two Union Pacific employees out of the wrecked engine, which became lodged next to the train's second engine. That engine began burning after the crash.

"We're very fortunate he was there," Walter said. He said all seven people injured were already out of the wreckage when he arrived about 15 minutes after the crash was reported.

"People were talking; they were coherent. They understood what was happening," Walter said.

The cars on the overpass "took a really bad hit" when they collided with the bridge sections, but "they stayed on all four tires and they just hit and landed and that was it," he said.

The accident came more than a week after a commuter train derailment in Connecticut that injured 70 people and disrupted service for days. That accident involved a railroad used by tens of thousands of commuters north of New York City.

In Washington state this past week, a bridge collapsed when a truck driver's load bumped against the steel framework.

NTSB board member Robert Sumwalt said "there is no similarity" between the Missouri accident and the bridge collapse in Washington, which sent two vehicles and three people falling into the chilly water. "This was a rail collision," he added.

The Union Pacific train involved in the collision was carrying primarily automobiles or auto parts from Illinois to Texas, said UP spokeswoman Calli Hite. She said about a dozen UP railcars derailed.

Hite said there was no estimate yet on the amount of damage to the roadway or the rail cars.

BNSF spokesman Andy Williams said about 12 cars on the 75-car BNSF train derailed. The BNSF crew was not hurt.

Sumwalt said NTSB investigation will include testing railroad employees for drugs and alcohol, testing the track and nearby rail signals and reviewing video footage from the front of the train in an effort to determine the likely cause. The NTSB will also review the bridge's design.