All trains to and from London's Waterloo were cancelled for two hours on Friday morning, causing major disruption for commuters.
Overrunning engineering work has been blamed for the disruption to the UK's busiest railway station.
No South Western Railway (SWR) trains were able to serve London Waterloo from 5am.
All lines were reopened two hours later, but delays and cancellations are expected to continue throughout the day.
South West Rail apologised to travellers for the disruption. "Services are being delayed due to over-running engineering work, sorry for the inconvenience this causes you this morning."
A spokesman for Network Rail, who were carrying out the engineering work, said: "We apologise to passengers for the disruption to services across the South Western network this morning owing to multiple possession overruns as a result of a power supply issue.
"Passengers should check with their train operator or visit www.nationalrail.co.uk before travelling."
Around 260,000 people travel through London Waterloo every day.