The Boston Fire Department said 23 people were taken to hospitals with injuries, none of which were life-threatening.
The cause of the crash was not immediately known, Massachusetts Bay Transit Authority General Manager Steve Poftak said at a press conference.
“We will obviously get to the bottom of this. This should not happen and we will find out why it happened and will ensure that it won’t happen again,” he said
The trains crashed on the Green Line track running along Commonwealth Avenue west of Boston University.