The power had to be turned off on a railway line into the UK's busiest station after passengers reportedly got off a delayed train and walked along the tracks.
South Western Railway services to London Waterloo were already disrupted because of a points failure in the morning rush-hour.
A train from Guildford to Waterloo was halted at New Malden and commuters were told the power had been turned off because passengers had got off a train in front and were walking down the tracks.
"It's a serious incident," a member of the train crew said.
There was major disruption to services to and from Waterloo.