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.
There are 15 minute delays on trains running between Stevenage and King's Cross because of emergency engineering works taking place at New Barnet. Some services from Moorgate to Welwyn Garden City are also being diverted.
M11 Southbound busy but moving due to fog between J7 A414 / B1393 (Harlow) and J6 M25.