London's underground Holborn station evacuated during Tuesday rush hour after commuters reported a loud bang while smoke fills platform.
The fire service said it had sent two fire engines and 10 firefighters to the station while police said officers were investigating the reports of smoke on a platform.
A witness on a train at the station said smoke filled one of the carriages.
"There was a fire alert on a westbound Central line train," a spokesman for London's transport operator said. "We are investigating the cause."
The station was reopened for service roughly after one hour it was evacuated, the transport operator said.