Network Rail has apologised to thousands of passengers facing long delays as a result of overrunning engineering works.
Commuters using the West Coast Main Line have continued to experience severe disruption today after services were brought to a near-standstill yesterday.
The national train operator says the problem arose because it did not have sufficient numbers of skilled workers to complete work at Rugby by the original deadline of New Year's Eve.
"The critical issue is a shortage of specialist engineering and contractor staff necessary to rebuild the overhead electrification," a statement from the firm said after admitting the route will not reopen today.
"Network Rail has brought in additional manpower from across the country to get the project completed and we continue to work flat out to restore a full service as soon as possible."
A replacement bus service between Northampton and Birmingham International will remain in place, having been running since December 27th.
Robin Gisby, Network Rail's director of operations and customer services, said: "We are extremely sorry for the continued disruption to passengers, which is down to the shortage of some critical resources and which will lead to a thorough and urgent investigation.
"We are working around the clock to restore services as soon as possible and apologise for the unacceptable delays while this work is completed."
Travellers in the east of England have also experienced problems during the new year return to work, with Liverpool Street station in London closed because of staff shortage.
Network Rail says all services from the east London station have now restarted.
The Office of Rail Regulation is now investigating the delays and is likely to fine the firm, while MPs have condemned the ongoing disruption.
Liberal Democrat transport spokesman Norman Baker said the situation was a "fiasco" and shadow transport secretary Theresa Villiers said passengers were on the "end of a real double whammy".
"Passengers deserve a lot better than the treatment they are getting," she said.