London Fire Brigade said 5 per cent of one carriage was alight on platform six on Charing Cross Station. No one was injured in the blaze.
Inspector Andy Hubbard from British Transport Police said: "It appears there was a defect with the ceramic pots beneath the track, which caused the electrics to short. We're ruling out anything criminal or terrorist-related as the cause."
Around 100 passengers were ordered to leave the station after the blaze started in the front carriage of a South Western train.
Five fire engines were dispatched, with 35 fire fighters and officers from the London Fire Service (LFS).
Rail passengers posted dramatic images of the blaze on Twitter, in which the carriage appeared to have been engulfed in flames.
The Underground station was closed for about 15 minutes before reopening, Transport for London said.