London's Holborn underground train station was closed on Tuesday as the emergency services responded to a fire alert and reports of smoke, the transport operator said.
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."