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
Abellio is to sell 40% of its Greater Anglia rail franchise to Japanese company Mitsui,
Three people have been identified and work is continuing to trace two others after CCTV images were released on the Corrie McKeague enquiry
Workers at a storage company in Suffolk had a surprise when they opened a shipping container and found a huge spider inside.