Flames and smoke began pouring from the southbound Orange Line train at around 6.45am as it approached Assembly Station in Somerville, Massachusetts, with video showing some of those on board kicking out windows to escape.
Massachusetts Bay Transportation Authority (MBTA) told The Independent via email that approximately 200 passengers were walked off the train by staff but some people did evacuate through windows in the first car.
“Public safety personnel picked up a person who had jumped into the water after exiting the train,” the statement said, adding that the person declined medical attention and no other injuries were reported.
The train had been travelling over the Mystic River when the incident occurred.
Power was cut to the line and the fire department responded. The train is being examined at a nearby railyard and the MBTA Track and Power Departments are at the scene inspecting infrastructure.
Boston is under a heat emergency through Thursday amid three consecutive days of temperatures above 90 degrees Fahrenheit (around 32C). A high temperature of 93F (33.9C) is forecast for Somerville on Thursday with 51 per cent humidity.