Warning over £8.6bn rail upgrade

But Westminster's Committee of Public Accounts (CPA) warned in a report that demand on the West Coast Main Line was growing so fast that it may reach capacity in just eight years.

Spending on the project was also being done on a "heroic scale", the MPs said, with the budget now standing at £6 billion more than original estimates.

Work on upgrading the line between London and Scotland via Carlisle - scene of a train derailment earlier this year which killed one passenger - was finished in late 2005 after several years of delays.

The project enabled high-speed Pendolino tilting trains to run on the tracks, slashing 50 minutes off London to Glasgow journey times.

Modernisation of the 400-mile route was originally intended to increase capacity over the next 20 to 30 years.

The CPA report said the line was still prone to overcrowding on peak services, and is likely to need further investment to cater for expected passenger growth.

It also called for Government, rail regulators and train operating firms to explore "every measure" they can to increase train capacity, make greater use of tracks and minimise disruption.

Committee chairman Edward Leigh MP (Tory, Gainsborough) said Network Rail had improved control over costs but there was "ample" room for improvement. He also criticised fare pricing structures, saying they could be complex and inaccessible for people without internet access.

Hourly seating capacity on the line is expected to reach 8,000 next year, up from less than 5,000 in 2004, the report said.