Trains are unable to leave or arrive at Victoria station in London after the fire on a Gatwick Express service.
Tom Edwards, an antiques dealer from London, who was trying to get a train to visit his mother, said he spotted flames and black smoke coming from the train.
"There was just a little bit of flames by the wheels," he said.
"They put it out and then started chucking everybody out of the building.
"There was no panic. It could have got pretty bad if they hadn't done something about it straight away."
London Fire Brigade confirmed a train was on fire and said crews were attending.