Two of the country's busiest train stations faced hours of delays and cancellations on Wednesday, in what one commuter labelled the "worst transport chaos" he had seen.
Hundreds of passengers struggled to get home from Victoria and London Bridge stations following a "major signal failure" near East Croydon.
Images posted on social media showed passengers crowded on the Victoria concourse, unable to catch their Southern, Southeastern and Gatwick Express trains home.
Across the city, Thameslink services out of London Bridge were also affected.
Rob Broomby, a TV producer, was stuck at Victoria and said it was the "worst transport chaos" he had experienced since the 1990s.
He added: "There was a lot of good humour in the bar as people settled in for a long wait, but when the platform indicators began flashing on and off it felt more like a Christmas tree with dodgy wiring."
Another of those stuck at Victoria was Labour MP for Hove and Portslade, Peter Kyle, who described the scenario as "not fit for modern Britain".
He apologised to customers and said staff would be "devastated" by the delays and cancellations.
He tweeted: "You deserve better.
"I'll continue fighting for the investment we need."
Earlier in the evening he had described the central London hub as "at a standstill" and said passengers had been "let down badly this evening".
Victoria and London Bridge are Britain's second and fourth busiest stations, carrying 75 million and 48 million passengers in 2017/18 respectively.
A picture posted on the Network Rail Kent and Sussex Twitter page showed the power failure affecting tracks from Streatham Common, east towards Penge and south towards Croydon.
Trains were running in the area at 8pm on Wednesday, but disruption was expected to last until the end of service.
Network Rail apologised and advised people to find other routes if possible.
