Firefighters dealt with a small blaze aboard a Bakerloo Line train at Oxford Circus station, and paramedics treated four people for smoke inhalation, taking two of them to a hospital as a precaution, authorities said.
Passengers said staff ordered everyone to leave the station just before 9 a.m. (0800 GMT) as smoke billowed from a train.
Passenger Tom Singer tweeted a photograph of smoke filling a carriage and spilling onto a platform. Another photo posted by Joe Bunting showed passengers in a smoky subway car covering their mouths and noses.
Subway operator Transport for London said the cause was an electrical fault under one of the carriages.
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