Buses will continue to replace Red Line train service between JFK/UMass and Park Street stations through Monday while crews continue to work in the tunnel, said MBTA spokesman Joe Pesaturo.
Crews are replacing and repairing damaged electrical and communications cables.
Firefighters responded to reports of a fire around 10:30 p.m. Saturday and entered the tunnel after power to the tracks was cut a short time later, Boston Fire Department spokesman Steve MacDonald said.
“The damage is extensive, and the work is complex,” Pesaturo said. “Crews are working tirelessly to have the Red Line fully functional for the Tuesday morning commute.”