Train passengers fled a platform after a power surge sent a flare 10 foot into the air from the track.
The flare pictured travelling along the line
An eye witness described people screaming as the fireball moved along the track at London Bridge station on Saturday afternoon.
Electrician Peter Difolco, 44, was returning from a shopping trip when he said there was a large flash.
"It was like a Roman candle firework, there was lots of smoke and people were screaming and running out the station."
Mr Difolco, who was with his wife, said he had an idea it was something electrical.
"I had guessed it was something electrical but there were a lot of frightened people.
"There was a small explosion like a firecracker. It went up quite high and started working its way down the track, it went about 120 foot and it lasted for about a minute.
"There were lots of police sirens and then the station was evacuated and staff were comforting people."
British Transport Police confirmed there was an incident involving a power line at the station at about 1330 BST.
The station closed for about 30 minutes.