A Houston Fire Department ambulance was making a turn at Fannin Street and Dryden Road when the METRORail collided with it, HPD said.
Police said several people on the train were hurt, and a firefighter who was in the ambulance had minor injuries.
Investigators estimated about five people were on the train during the crash, but did not have an exact number. HPD did not provide any information on the condition the passengers were in when they were taken to area hospitals.
Police said the ambulance was transporting a patient to the hospital at the time of the crash.
The METRORail red line runs through the Texas Medical Center.