1130 PM EDT - sounds like it is all under control now. The fire might have been in a transformer on the subway wall between Park Street Station and Downtown Crossing Station on the Red Line. Approx 20 people were taken to hospitals, but injuries seem to be minor.
TV7 News had a report (from a party that was on a subway car) that they saw a MBTA worker trying to fight a fire on the wall of the subway tunnel with a fire extinguisher. That seems to jive with the report of the transfomer fire.