When two power stations disconnected from the National Grid for 15 minutes, railroad travelers collapsed for hours.
How did it happen and why did it have such a big impact?
Why did the power stations fail?
It started with a routine blip ̵
Then, another power station, the new Windsea offshore wind farm, "also lost its load" – meaning that the turbines were still moving, but the power did not reach the grid.  Two power stations that close almost simultaneously are "a very rare event," says David Hunter, an energy analyst at Schneider Electric. "It surprised the national grid."
He states that an examination of the causes may show that the two errors were "accidental and unrelated", adding that there have been occasions when two generators shut down independently before.
But he said that a power station that runs off the grid can also create a "domino effect" where other generators strain under the load to make up for the lack of power.
Was the wind power to blame?
Despite strong winds on Friday, National Grids operations manager Duncan Burt said the closures had "nothing to do with changes in wind speed or the variation in wind."
Experts say there is no reason to believe that wind farms are more likely than other generators to disconnect from the grid.
But they say that renewable energy such as wind power can create problems for the national grid.
"Gas and coal power plants have greater flexibility. When one gets off, it is easier for another to catch up on the shortage," says Hunter.
Wind power is less efficient as a "shock absorber" to shift supply and demand, according to the energy analyst.
He said National Grid designed systems that were meant to cope with increasing amounts of renewable energy and the associated problems – but it was "too early to say" if it was a factor in Friday's power cut.
Why were parts of the web closed down?
The national grid must maintain a 50Hz electrical frequency. When supply falls and demand remains high, the frequency drops to 48.9Hz in this case, well below the accepted level.
Incorrect frequency can be dangerous, Hunter says, causing damage to the UK's electricity infrastructure.
National Grid states that their automated systems kick in to stop any damage, mainly by cutting off parts of the grid to reduce overall demand.
Stuck in a train without food or toilets
But Lord Adonis, a former chairman of the National Infrastructure Commission, has questioned the design of the automated systems if one of the first things affected was the national transport system.
He says the National Grid needs to make sure "when something like this happens – two power stations going down – it doesn't stop the whole country."
Why did it take so long to fix problems?
The blackouts were resolved relatively quickly. The power stations were back in action after 15 minutes and National Grid says local power suppliers met the demand by 17:40.
But there is little consolation for passengers like Lawal Brown, who boarded a Thameslink train in Stevenage at 4:45 p.m. .
Network Rail said that darkness affected signaling and power supply equipment across much of the rail system, but backup systems intervened.
There were still some delays due to security requirements, said Nick King, Network Services Director for Network Rail.
But he says that further difficulties were caused by a "major system failure" on "a particular train fleet". . Thameslink has acknowledged that the trains required a technician to restart them after a power cut.
Mr King says this caused "significant disruption" to parts of the network, although Network Rail, train operators and police worked "flat out" to rescue stranded passengers.