The first car of an outbound Riverside trolley derailed shortly after 12:30 p.m., hitting a tunnel wall.
Boston EMS reports ten people injured.
Three people with injuries were able to walk out of the tunnel near Fenway station; a third was put on a backboard to be carried out by firefighters. The rest were put on the rear trolley, which was disengaged from the front car and backed up to Kenmore station.
Among the injured: The driver of the first car, who was able to walk out through the Fenway tunnel portal.
The derailment happened just after Kenmore Square, where the D Line splits from the C Line.
Service on both the D and C lines was affected.