Track posessions at Rugby were due to complete on 1 January, but the track remains closed between Birmingham International and Northampton and will reopen on Friday morning.
Network Rail's chief executive, Iain Coucher, has demanded an explanation for the delays from its lead project managers on the project - Bechtel. Network Rail could face a large fine from the Office of Rail Regulation for the delays, which have inconvenienced thousands of commuters and other travellers.
"I can confirm that trains will start to run normally on the West Coast again from tomorrow," said Coucher.
"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. I know that we have let passengers down with the over-run at Rugby.
"An investigation into the causes of the over-run has been launched and I will be chairing a high-level meeting next week to review what's happened. 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.
"The railway will be back tomorrow but the work is far from done. We have a significant programme for developing and expanding the railway over the coming years and we have to make certain that it is carried out to plan."
Rugby was the centre of significant works over the Christmas to New Year holiday period.
- Installing over 5km of overhead line
- Bringing into use 5.3km of new track
- Bringing into use 15 new sets of points
- Installing two new sets of points
- Bringing into use 18 new signals
- Making 16 new track connections
- Demolishing a mini-signal box
- Removing 11 sections of overhead wire and moving a further 14
- Removing four signalling gantries
- Installing nine new signalling 'booster' cabinets
These improvements were part of works to remove the Rugby 'bottleneck', which significantly reduces journey times between London, Birmingham and Manchester.