The Strategic Rail Authority said the 9.8-billion-pound ($15.3 billion) project would be mostly completed by 2006.

It will provide capacity for 80 percent more long distance passenger trains, but those trains will only be allowed to run at 200 km (124 miles) per hour, below the 225 km per hour originally envisaged.

"Today's announcement is about the renewal of 780 miles of railway and the elimination of a 20-year maintenance and renewal backlog in the shortest practical time," SRA Chairman Richard Bowker said in a statement.

Britain's Transport Secretary Alistair Darling told BBC Radio: "What we are doing here is being realistic about the state of the railways. This line was last done up seriously in the 1960s, 40 years ago. We are paying a very heavy price for years and years of under-investment, but we are now getting a grip of it."

Failure to upgrade the line amid spiraling costs was a key factor behind the demise of Railtrack, the collapsed privatized rail infrastructure company recently superseded by the not-for-profit Network Rail.

Network Rail, owned by train operating companies, passengers, unions and other interest groups and backed by the state, plans to tap international debt markets to finance rail investments.

Darling was scathing about Railtrack. "All in all, you now have a realistic program...which can actually be delivered. Frankly a lot of Railtrack's proposals were pie in the sky."

The Rail Passengers Council, a consumer organization, was upbeat about the SRA's announcement.

"We have said that it is imperative that the industry gets a grip on the West Coast upgrade -- spiraling costs and delays do little to inspire passenger confidence in our railways," said Stewart Francis, the council's chairman. "Today's costed and specified plan is to be welcomed and I think passengers will be relieved to hear that someone is taking charge of the project."

The decaying railway, and the consequent delays and deadly crashes, had enraged many riders in recent years.

The upgrade will cut journey times from London to Glasgow, Scotland by 45 minutes. The line has spurs to key English cities such as Birmingham, Manchester and Liverpool.

The SRA said passengers would see improvements from autumn 2004. There will be four trains an hour to Birmingham from London in peak hours, up from two trains now, and three trains to Manchester. Freight capacity will rise 60-70 percent.

The upgrade is part of government plans to cut car use and ease congestion on motorways, notably the M6 running through Birmingham in the midlands, which more often resembles a car park in peak hours than a main road artery.

But that traffic bottleneck is also being improved and Virgin Trains, the main train operator on the west coast line, will face competition from a relief road being built north of Birmingham and due to open in early 2004, six months ahead of the improved train timetable.

A Virgin Trains spokesman told Reuters while it welcomed the SRA's proposals, its new generation of Pendolino trains had been bought with the intention of running them at 225 km per hour.

"We shall continue to campaign for a signaling system which will allow trains to run at this speed eventually," he said.

The west coast upgrade could also result in the fastest journey times from Northampton, a town in the south midlands set to become a key commuter terminus, being cut to 50 minutes from an hour, the SRA said.

Northampton and nearby Milton Keynes, which already has an express rail link with London, have been earmarked by the government for major expansion over the next 20 years to accommodate key workers -- nurses, teachers, emergency service workers -- unable to afford housing in the affluent south-east.

To help deliver the new train service, sections of the line are closed completely at weekends, with passengers having to transfer to coaches for part of their journey.

Virgin Trains said over 90 percent of passengers were happy with its management of the train/coach/train journeys.