Recent overhead power problems in Hertfordshire and Cambridgeshire which led to rail chaos were caused by "component failures", Network Rail has said.
The company said it had identified exactly which components failed at Radlett in Hertfordshire and at St Neots in Cambridgeshire.
Network Rail has apologised to travellers who were caught up in the delays.
"We are carrying out further work to establish the reasons so that we can minimise the chance of repeat failures in the future," said a company spokesman.
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Flights have been cancelled, diverted and delayed after a lightning strike overnight knocked out Stansted's aircraft fuelling system.
Sunny spells. Very warm inland. Isolated thunderstorms. Cooler near coasts.
Mostly dry overnight. Some warm sunshine Sunday, however thunderstorms possible.