The train derailed after a recently repaired bridge collapsed over boggy ground Wednesday, NASA spokeswoman Katherine Trinidad said, adding that six people aboard were reported injured, one in critical condition.
She said eight booster segments were on the train. One booster overturned, along with two locomotives and a car carrying six attendants, who were injured.
NASA said the segments were not slated for use during the next shuttle flight, the liftoff of Atlantis June 8, but for missions in October and December.
It said NASA's solid rocket boosters and their parts are freely interchangeable.
The space agency said the derailment was in a remote stretch of track near Myrtlewood, about 110 miles southwest of Birmingham.
NASA said earlier it is planning to launch four space shuttles before the end of the year.
NASA managers had been pondering whether to finish repairing a tank and use it for Atlantis' mission or replace it with another tank. But they said they were pleased with the progress of repairs that have been made.
They said Atlantis would return to the launch pad May 6, after its hail-damaged fuel tank has been repaired, but that the first launch will not be made until June 8.
Atlantis was to have lifted off in March, but the launch was put on hold after golf ball-sized hail caused thousands of dings on the fuel tank's foam insulation.
The fuel tank has about 2,500 dings, 1,600 of which may need to be filled in with new foam.
