Tens of thousands of rail passengers from Kent, Sussex, Hampshire and Surrey have been caught up in this morning's incident at London Victoria.
Southern, Southeastern and Gatwick Express services use the station which is one of the busiest in the country.
The station was evacuated at one if the busiest times of the day.
Some passengers were left stranded on trains for more than an hour.
There are delays to all routes and problems will continue for much of the morning.
Train operators say delays and cancellations could last for a couple of hours.
Southern trains spokeswoman Jane Lee said staff spotted smoke coming from under a Gatwick Express train as it came into platform 13.
Fire alarms were sounded and the station evacuated as a precaution.
Fire crews were called who dealt with the incident.
They waited for smoke to clear and the station opened again at 0745.
An inquiry is underway into the incident.
An electrical fault with a Gatwick Express train is thought to be the cause of this mornings incident involving at Victoria.
There are no injuries.
Overheated brakes or a train motor are at the centre of initial investigations.
Staff spotted smoke coming from under the train that was empty.
There were reports of large amounts of smoke.
A train fire has caused delays at London Victoria station.
Because of this, the station has been evacuated and trains are unable to run to and from the station. Journey times may be extended by up to 45 minutes.
Southern passengers may use:
Southeastern between London Bridge and London Charing CrossSouth West Trains between Clapham Junction and London Waterloo
First Capital Connect between East Croydon and London Bridge