Part of a key rail line reopened on Friday after engineers finally finished late-running engineering work that disrupted tens of thousands of journeys, Network Rail said.
Work on part of the busy West Coast Main Line was due to finish on New Year's Day, but a lack of specialist workers led to long delays.
A review is now being undertaken after Network Rail's Chief Executive Iain Coucher admitted his company had failed passengers.
"I can take no comfort from this news in the knowledge of the pain and inconvenience we have caused passengers over the past few days," he said in a statement. "I know that we have let passengers down with the over-run at Rugby."
Coucher said the project's managers, Bechtel, and the principal contractors would be summoned to a meeting next week to explain why the work took so long.
"They gave me assurances before Christmas on which they have not delivered," he added. "I want answers from them."
Virgin Trains, which runs services on the line - and earlier advised passengers not to use its routes on Thursday - welcomed the news. A spokesman said it planned to run full services.
"Of course we are pleased - we were wanting to run full services from the 31st of December and we would have had it not been for these over-run engineering works," he said.
The Office of Rail Regulation, the industry watchdog, has launched an urgent investigation into the problems at Rugby.
