The power failure which hit a Network Rail operations centre in Manchester was a rare incident.
But after commuters were left facing chaos, it has raised questions about how a short outage at one operating centre could have a widespread impact.
Network Rail will face scrutiny for how its systems coped and responded to the outage.
That's the publicly-owned body whose job it is to operate and maintain Britain's railway infrastructure. That means the tracks, signals, embankments and major stations.
It was at Network Rail's operating centre in Manchester that the problem started. Its operating centres are where crucial controls are housed that let trains run safely across the country, with Manchester's centre responsible for large parts of the North West.
It was hit by a power cut in the area, which was resolved within 90 seconds. Given the centre's importance, backups are in place. But questions have been raised about how they responded.
Chris Wright, north-west route director at Network Rail, told the BBC "we didn't experience a 90-second interruption," insisting its systems were back online sooner.
But he said some of Network Rail's communications systems "did not respond how we would have expected, and then needed reconfiguring and replacing".
"We've got to work really hard very quickly now to understand why that's the case," he added.
Because signalling was affected – think of that essentially as the traffic lights of the railway – trains had to stop.
Systems had to be rebooted. But there was some damage to those that control signalling, which took time to fix because of the need to replace certain parts.
Thinking about how busy parts of the rail network are, and how many trains and crews were left out of position across the North West of England and beyond, helps explain why the impact was so big.
Network Rail has apologised to customers and opened an investigation.
Bosses say engineers have been working hard to "build additional resilience into our signalling systems to ensure that this doesn't happen again".
Network Rail has moved towards having 12 so-called Rail Operating Centres across Great Britain, replacing a previous structure of hundreds of older signal boxes. They are meant to be high tech hubs which centralise control of the network.