MYRTLEWOOD, Ala. — A train carrying segments of the space shuttle's solid rocket boosters from Utah derailed Wednesday after a bridge sank over boggy ground, authorities said. Six people aboard the train were reported injured, one critically.

NASA said it was not immediately known whether the equipment was damaged. But space agency spokesman Allard Beutel in Washington said the accident should not delay any shuttle launches.

The trestle collapse was under investigation.

The shuttle's twin boosters are 150 feet tall and consist of four propellant segments each. They are used during liftoff and the first two minutes or so of flight to help the spacecraft break free of Earth's orbit and are then jettisoned into the sea, after which they are recovered, refurbished and reused.

It was a leak of burning gas between two segments of a solid booster rocket that caused the Challenger explosion that killed seven astronauts in 1986.

"It appears when the train got onto the trestle, the trestle just gave way and sank to the ground," said Mike Rudolphi, an official with the boosters' manufacturer.

Eight booster segments were on the train, which carried only the shuttle shipment, Rudolphi said. One booster overturned, along with two locomotives and a car carrying six attendants, who were injured.

The segments will probably be sent back to the manufacturer's Utah plant to be examined, Rudolphi said.

The Alabama Emergency Management Agency said the four derailed train cars landed on their side on the ground in the woods next to the tracks, none of the cargo spilled, and there appeared to be no fire.

Twelve rail cars remained upright. It appears those cars didn't overturn because the trestle — about 300 to 400 feet long — sank evenly to the ground, Rudolphi said.

The derailment was in a remote stretch of track near Myrtlewood, about 110 miles southwest of Birmingham. Sheriff's Deputy Kenneth Collier said he was unaware of any threat posed to the public.

"You can't get any further out in the woods than this," he said.

NASA spokesman Kyle Herring said the segments were not scheduled for use during the next shuttle flight, the liftoff of Atlantis on June 8, but for missions in October and December. NASA's solid rocket boosters and their parts are freely interchangeable.

It was the second time in less than a week that the train jumped the tracks while carrying the booster segments across the country from the manufacturer, ATK Launch Systems Group of Promontory, Utah, to Cape Canaveral, Fla., NASA said.

Last Friday, two axles on one car came off the tracks for unknown reasons about 60 miles west of Salina, Kan., while the train was traveling at less than 20 mph, Herring said. The train was back on the tracks after several hours, the spokesman said.