The West Coast Main Line has fully reopened today after services were disrupted for four days because Network Rail did not complete the work on time.
Network Rail has apologised for the delays and chief executive Iain Coucher is launching an investigation into the incident.
Mr Coucher said: "I will be summoning our project managers, Bechtel, and the principal contractors involved to attend so they can personally explain to me what has gone wrong.
“They gave me assurances before Christmas on which they have not delivered. I want answers from them.”
Bechtel had eight contractors, including Jarvis, working on the track and it is reported that insufficient engineers were the cause of the delay.
The network operator is now facing an investigation from the rail regulator, which is expected to lead to huge fines.
An estimated 60,000 passengers had to contend with badly-disrupted journeys again today with the line shut in the Rugby area of the West Midlands.
There are also calls for Network Rail to bring its renewal works back in-house in order to gain greater control of its lines.
General secretary of the Rail Maritime and Transport Union Bob Crow said: “"Network Rail has already shown how much better things can be by bringing most routine maintenance back in-house, and there is no earthly reason why the bulk of major renewals contracts shouldn't come back in-house as well."