Network Rail launched an investigation and offered an apology as rail passengers faced at least another day of travel misery today after its failure to complete major engineering work on the West Coast Main Line.
The work in the Rugby area should have been finished in time for the start of services on Monday this week.
But blaming "a shortage of specialist engineering and contractor staff", NR was unable to reopen the West Midlands section of the line yesterday for the return to work after the festive period.
As passengers spoke of "nightmare journeys" which were compounded by a fire which caused an evacuation at Birmingham International station, opposition MPs joined in the condemnation of NR, while the Office of Rail Regulation ORR said it was launching "an urgent investigation" into the West Coast situation.
The ORR said it was also looking into the overrunning of NR engineering works which meant that Liverpool Street main line station in London was closed yesterday, affecting services from the capital to East Anglia.
The £400 million track and signalling work at Rugby is part of Network Rail's £6 billion plan to refurbish the West Coast Main Line by the end of the year. It means all trains usually passing through the town have been replaced with bus services.
Passengers had to take coaches between Northampton and Birmingham International stations on the busy route between London, Birmingham, the North-West and Scotland.
NR operations and customer services director Robin Gisbey said the overruns were "completely unacceptable" and added that an investigation into what had gone wrong had started.
Mr Gisbey said he was "deeply sorry" about the further disruption to passengers.
The travel misery was compounded by the fact that this was the day that new higher-than-inflation train fares came into force.
Shadow Transport Secretary Therese Villiers said passengers had been "on the end of a real double whammy" and added that Labour
had "let rail passengers down badly".
Liberal Democrat transport spokesman Norman Baker dubbed the overrunning "a fiasco".
There was further condemnation last night from Virgin Trains which complained to the rail regulator.
Virgin Trains chief executive Tony Collins said: "This has been a fiasco and our customers expect and deserve better.
"We are pleased that the Office of Rail Regulation is investigating this urgently."
Virgin said it was advising its customers not to travel on its services today. It added that NR had "assured Virgin Trains that it is likely that the West Coast Main Line will reopen at Rugby on Friday".
Virgin also announced that saver fares would be available on all trains, including peak services, next Monday and Tuesday, to enable customers to travel at cheaper prices at peak times.
It added that customers holding Virgin Trains tickets dated for today who decided not to travel could claim a full refund from where they purchased the ticket.