The cost of the long-awaited West Coast Main Line modernisation has been slashed by £5.4bn.
The work has been beset with delays and increasing costs
The Strategic Rail Authority (SRA) said the predicted £13bn upgrade of the line will now cost £7.6bn.
The much delayed upgrade, which includes introducing new 125mph tilting trains and reduced journey times, should now be completed by 2008.
Rail bosses confirmed on Tuesday that faster journey times between London and Scotland will start in September.
Spending on the modernisation had already been reduced to £10bn by the SRA.
It said the planned cut in journey times from London-Edinburgh from five hours six minutes to four hours 40 minutes had been finalised.
It is hoped to reduce the time by a further 25 minutes by December 2008.
The authority said work on bottlenecks at Rugby and the Trent Valley would be completed by 2008, later than expected but earlier than that suggested by the regulator.
SRA chairman Richard Bowker said: "As set out in our June 2003 strategy, passengers and freight customers will see a step change in performance on the West Coast Main Line from September 2004, with more to come in 2005 and 2008.
"The industry is working night and day to deliver improvements across the entire route - North to South - to ensure this national asset is fit to deliver for customers and for the UK economy."
Progress on the upgrading the track and signalling has been hindered by delays and spiralling costs on a previously cash-starved line.
The Pendolino trains, already in use on part of the line, tilt round bends making them quicker than older trains.
Virgin West Coast, who will run the trains, is bringing back a restaurant car to its London to Glasgow trains from 27 September.
It will be available to passengers travelling first and standard class.