The West Coast main line will be unable to cope with increasing passenger traffic within 15 years, a new report says, despite a total investment of £8.6 billion.
But government policy watchdog the National Audit Office (NAO) says that the series of upgrades agreed upon in 1998 to improve customer services on Britain's busiest line have ultimately proved successful.
Today's report says that Network Rail and the Department of Transport were responsible for the turnaround after a series of management errors by defunct privatised firm Railtrack.
In response to claims that the line connecting Scotland, the north-east of England and London, will be unable to cope with passenger demand by 2020, Network Rail insists that it has contingency plans to deal with such traffic rises.
Its deputy chief executive the Today programme that the non-profit group had been aware of the predicted increases.
"Going forward it will continue to rise and that's a measure of our success really. We've got a programme of work still being undertaken now and more trains will be coming on over the next couple of years," said Ian Coucher.
"And going forward there's a whole range of things we can do. Longer trains, more trains per hour through the key bottlenecks and then eventually closer together through new signalling systems," he added.
The NAO today criticised the management of the improvement project for investing in unproven technology, but praised the line in its present form for cutting journeys from Manchester to London and Glasgow to London by 36 and 42 minutes respectively.
Sir John Bourn, head of the auditing office, commented: "When the Strategic Rail Authority stepped in, the project to modernise the West Coast main line was in disarray, vastly over-budget and with few of the planned improvements in place.
"It was only through good direction by the Strategic Rail Authority and then the Department for Transport and through the exercise of firm management by Network Rail that the project was brought back on track so that benefits of faster journeys are now being delivered to passengers."
Sir John said that Railtrack's many errors should act as a warning for future projects.