The Texas Eagle, which runs between Chicago and San Antonio derailed around 11:30 p.m. Wednesday while carrying 90 passengers and 13 crew members, Amtrak spokeswoman Vernae Graham said.
The train was moving slowly through a winding area near Blackwell, about 50 miles southwest of St. Louis, when it apparently struck the rockslide, Jefferson County Sheriff Glenn Boyer said Thursday. He described boulders about half the size of a car hood on the track.
"We're fortunate the engineer had to slow down," Boyer said. "It probably could have been a lot more serious."
The cause of the slide was not known, but the area had received about 1 ½ inches of rain earlier Wednesday.
The engine and its six passenger cars left the tracks.
The injured passengers were taken to a hospital while others were taken by bus to a firehouse in nearby De Soto to spend the night. Most of those injured suffered neck and back pain, Boyer said.
It wasn't clear how long the uninjured passengers would remain in the town, or how long the track would be closed.
The National Transportation Safety Board planned to investigate.
Earlier Wednesday, a high-speed Amtrak Acela train at a crossing in Waterford, Conn., killing a woman and her 8-year-old grandson and critically injuring her 4-year-old granddaughter. The crash caused major delays along the Boston-to-Washington corridor.