A train pulling eight space shuttle booster segments, each loaded with solid propellant, derailed in Alabama today when a bridge collapsed, leaving two locomotives and one segment car on their side. Six people on the train were injured, two seriously. Engineers with booster-builder ATK Thiokol were on the way to the scene to assess the condition of the motor segments, the first step in ultimately determining whether they are safe to fly.

"Several members of the NASA family were injured in this serious accident," shuttle Program Manager Wayne Hale said in a NASA statement. "Today our prayers are for those who have been injured and their families. Our employees work in hazardous jobs every day, and it is our goal to keep them safe."

The train, carrying only booster segments and a passenger car for workers who accompany the hardware, was crossing a bridge when the trestle collapsed. "Six people were injured when the two locomotives and the passenger car dropped about 10 feet and turned on their sides," NASA said in a statement released late today.

"One of the cars carrying a solid rocket motor segment is also on its side. The remaining cars containing seven solid rocket motor segments and two aft exit cone segments are upright."

The same train suffered a minor, unrelated derailment last Friday in Kansas, officials said, but today's incident was much more serious. Even so, a NASA spokesman said the rubbery propellant in the booster segments does not easily ignite and there was little risk of fire or explosion.

The space shuttle uses two solid-fuel boosters and three hydrogen-fueled main engines to push the orbiter into space. Each booster is made up of four 300,000-pound fuel segments that are shipped loaded, one segment per enclosed rail car, to the Kennedy Space Center from Thiokol's Utah manufacturing facility. Once in Florida, the segments are inspected and bolted together. After two boosters are "stacked" on a mobile launch platform, an external tank is attached and then a shuttle.

The boosters are the largest solid-fuel rockets ever flown and the first ever used for manned space flight. Each four-segment "SRB" generates 2.6 million pounds of thrust and weighs 1.3 million pounds. The SRBs provide the initial thrust to get the shuttle off the launch pad and only operate for two minutes before exhausting their propellant, detaching from the shuttle and parachuting into the ocean for recovery and eventual reuse. The eight segments involved in today's derailment are slated for use by the shuttle Discovery in October for mission STS-120 and by the Atlantis in December for mission STS-122. The goal of STS-120 is to deliver a new connecting module to the international space station that will permit the attachment of Europe's Columbus research module during STS-122.

"These segments are interchangeable, and ATK Launch Systems has replacement units that could be used for the shuttle flights, if necessary," NASA said in a statement.