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.
More top news
Three men caught on CCTV assaulting a man in the violence following last year’s Luton Carnival have been given jail time.
We look ahead to a big weekend in League Two as the battle for the play-offs intensifies.
Southend United manager Phil Brown will reach four years at the club when his side visit AFC Wimbledon on Saturday.