Network Rail's chief executive tonight attempted to reassure furious commuters that over-running engineering work in the Midlands would finally finish by tomorrow morning – a total of five days late – as chaos continued for passengers using a key train route.

In a statement issued tonight, Iain Coucher added that he was summoning the contractors involved in the work to see him so they could "personally explain" why they had failed to meet their deadline to finish the project on the West Coast main line at Rugby by New Years Eve.

The delay has meant customers having to take coaches between Birmingham and Northampton, on one of the UK's busiest train routes.

As a result, journeys have taken around two hours longer than normal and Network Rail has been sharply criticised by Virgin Trains, which runs the train route.

The company faces being fined millions of pounds by the Office of Rail Regulation over the delays. In total, they affected an estimated 200,000 people, with further overrunning work also closing London Liverpool Street mainline station yesterday. London's second-busiest station eventually reopened this morning.

Speaking tonight, Mr Coucher reiterated the company's previous apology to passengers affected by the chaos, and promised to investigate with contractors why it happened.

"I can confirm that trains will start to run normally on the West Coast again from tomorrow," he said.

"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."

Network Rail has been roundly condemned by MPs, passenger groups and business concerns over its handling of the festive engineering work.

The disruption has caused particular anger as it coincided with above-inflation train fare rises taking effect, leading to threats by thousands of passengers to stage a fares strike, as revealed earlier this week in The Times.

Today, the Government came in for criticism for the first time over its handling of the affair, with the Conservatives saying that Labour Ministers – as the creators of Network Rail – should take a share of the responsibility.

"Labour created Network Rail. They have as much control over it as ministers did over the old-style nationalised industries of the past. Ruth Kelly (the Transport Secretary) needs to carry the can for NR’s failure," Theresa Villiers, the Shadow Transport Secretary, said.

"NR has already been fined for this sort of overrun. It sadly shows it hasn’t learnt from its past mistakes. Ruth Kelly should be taking action to make NR much more accountable and transparent to passengers."

A Department for Transport spokeswoman said it that the Office of Rail Regulation investigation should be allowed to run its course.

"While these engineering works are intended to deliver improvements, like many passengers we are very disappointed they have over-run into the working week. We are talking to Network Rail about what they are doing to complete this work," she said.

"The Office of Rail Regulation was created to scrutinise Network Rail's performance standards. The ORR has already announced an urgent investigation into engineering delays on the West Coast Main Line.

"As a result, passengers can be confident that these delays will be thoroughly investigated and action taken if appropriate."

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