Cpl. Justin Dunn, a spokesperson for Missouri State Highway Patrol Troop B, told reporters that two of the people who were killed were aboard the train while the third was in a dump truck that the train struck, CNN reported.
Authorities said the tragic incident happened at an uncontrolled intersection -- without warning lights or motion gates -- where a gravel road crossed the railroad tracks southwest of town.
Amtrak said the train collided with the dump truck at a public crossing near the city of Mendon at about 12:42 p.m. CT (1:42 p.m. ET).
Eight cars and two locomotives left the track "after striking a truck that was obstructing a public crossing near Mendon, Missouri," company officials said in an updated statement. Dunn told reporters that seven cars had derailed.
Amtrak had said earlier that there were approximately 243 passengers and 12 crew members aboard the train.
A school has been turned into a triage center for victims with minor injuries, officials said.
RHM/PR