Thousands of homes in southeast London were cut off from power on Sunday morning after a fire at an electricity substation that also forced the closure of a nearby rail line.
The fire damaged two electrical transformers, said London Fire Brigade.
Six fire engines and 30 fire fighters brought the blaze under control shortly before midday.
EDF Energy cut supplies to homes connected to the substation at Forest Hill shortly before 9 a.m. for safety reasons, restoring power to the majority of those affected around three hours later.
Network Rail said train services passing the burning substation had been suspended on the advice of the Fire Brigade.
Services into London Bridge were affected, a Network Rail spokeswoman said.
Eye witness Lisa Brown from Sydenham Hill told BBC television she saw "huge black billows" of smoke from the fire.
She said traffic lights also lost power in the area, causing problems for drivers at junctions.
Scotland Yard said police had been alerted to the fire at the substation in Sydenham Park Road shortly after 7.20 a.m.
There were no injuries, a police spokesman added.
