Investigations are continuing into what caused two miles of overhead power lines to come down over a busy rail line in Buckinghamshire.
Services on the West Coast Main Line between Northampton and London were stopped on Thursday after the damage at Castlethorpe, near Milton Keynes.
Network Rail has reopened two of its four lines but fewer trains than normal will be running on Friday.
The disruption will affect Silverlink and Virgin services.
Train parts and cable sections are being examined by technical staff and the cause of the problem will be known early next week, Network Rail said.
A spokeswoman for Network Rail said the 0939 GMT Glasgow to London Virgin train was the first to be hit by the problems, at 1330 GMT on Thursday.
Passengers on the affected train were taken off and replacement bus services were organised.
A spokeswoman said: "Network Rail would like to apologise to passengers for this disruption and can assure passengers that it is doing everything it can to repair the damage and keep delays to a minimum."
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