A major power failure thought to have been caused by vandalism led to severely disrupted train services at Birmingham New Street during the morning peak.
Many services were either delayed or cancelled and Network Rail advised passengers to avoid the station unless their journey was "absolutely necessary".
The failure of the power supply to the signals - which control the movement of trains - meant services could not enter or exit the south end of the station.
Virgin Trains, London Midland, Arriva Trains Wales and CrossCountry trains were affected.
Network Rail (NR) issued a photograph showing damaged cables with a message stating that "vandalism was likely cause of today's signalling failure".
An NR spokesman said: "Our engineers have fixed the problem at Proof House Junction, south of Birmingham New Street station, and trains will now start to return to normal.
"Network Rail's orange army has been working on the issues since the early hours of the morning to restore the power supply to the signalling in the area.
"This led to delays and cancellations as operators were not able to run trains in or out of the south end of the station.
"We are very sorry for this disruption and a difficult start to the week, and Network Rail is working with the train operators to get services back to normal as quickly as possible."
Services are expected to be disrupted until at least 4pm.
Passengers travelling on other routes through Birmingham were also warned that trains and stations were likely to be very busy.
Last week thousands of journeys were disrupted due to damage to overhead electric wires near Birmingham Airport.
The severe delays between Birmingham and Coventry affected services operating across much of the country, including Edinburgh, North Wales, Manchester, London and Southampton.