Firefighters dealt with a small fire on a train on the Bakerloo line at Oxford Circus.
The station was evacuated before the Brigade arrived.
The London Ambulance Service confirmed that four people were treated for smoke inhalation with two taken to hospital as a precaution.
The Brigade was called at 0847 and the fire was under control by 1035. Fire crews from Paddington and Lambeth fire station were at the scene. The cause of the fire is under investigation.