The station was evacuated at around 11pm with smoke billowing out of the tunnel.
London Fire Brigade station commander Keith McDermott, who was at the scene, said: “Firefighters investigated smoke coming from a tunnel and determined that it was caused by overheating in an insulating pot.
“Trains stopped running through the station whilst crews worked to make the scene safe. There were no reports of any injuries.”
The firefighters extinguished the blaze by 1.30am, however the Bakerloo line is still suspended between Elephant and Castle and Lambeth North.
A Transport for London spokesperson confirmed this was due to the fire.
TfL says: “No service between Elephant and Castle and Lambeth North due to emergency engineering work to fix damaged signalling equipment at Elephant and Castle.
“There is a good service on the rest of the line. Valid tickets are being accepted on London buses.”
The Northern line is still running at Elephant and Castle.