Smoke filled the air at the Broadway Red Line station Monday after a train experienced a mechanical issue, the MBTA said.
Trains on the line are now experiencing delays of up to 20 minutes, the MBTA said.
"An official is on the train working to move it to Andrew. Fans have cleared the air at Broadway due to earlier smoke," the agency said.
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The affected train was traveling southbound and is being taken out of service en route to the MBTA's maintenance yard.
It wasn't immediately clear what caused the issue, or if a fire had broken out.
Earlier Monday, the agency released the cause of the June derailment on the Red Line.