London Bridge mainline station was closed for about 30 minutes on Saturday after a power surge sent a flair 10ft into the air from the track.
"It was like a Roman candle firework, there was lots of smoke and people were screaming and running out the station," electrician Peter Difolco, told the BBC.
"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."
The incident took place at about 1.30pm on Saturday afternoon.
• Did you witness this incident? Send us your pictures.
For the latest local news and events direct to your inbox every Monday, you need our weekly email newsletter SE1 Direct.
7,000+ locals read it every week. Can you afford to miss out?
Read the latest issue before signing up