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
Thousands of people turned out to party in Luton as the town staged its 40th carnival.
Increasingly murky overnight. Dull start Monday, but sun breaking out later in places
A soldier from Essex is celebrating after becoming the first magician to win Britain's Got Talent.