London Overground runs passenger services on the line, which connects Stratford in the east to Richmond in the west and Clapham Junction in the south, on behalf of Transport for London.
The North London Line currently runs only two tracks through 11 stops on the section between Stratford and Gospel Oak and is now running close to full capacity.
The revised plans will mean four-tracking less of the line from Camden Road station to beyond Highbury & Islington station; the expansion will now start further east just before Caledonian Road & Barnsbury station.
TfL said the cost of bringing disused bridges around Camden Road station back into service is now unaffordable, and that it would also cause major disruption to busy roads in the area.
A source close to the project told Construction News: “There’s lots of civil work and track work associated with the job, and also lots of possessions booked for 2009. That means TfL need to scope the works so they can afford them and get them done in time.”
The job was first put out to tender at the start of the year with Carillion, Balfour Beatty, Colas and a joint venture of Grant Rail/Fitzpatrick believed to be bidding for the work.
It is now likely to be awarded in the first quarter of 2009, with construction due to start in the spring.
Under the new plans London Overground could run eight passenger trains an hour at peak times on the core section of the line between
Camden Road and Stratford - a key part of the 2012 Olympic transport plan.
A Network Rail spokeswoman said the change in construction plans would affect only off-peak services, which will involve only six passenger trains an hour rather than eight.
The job is the first major piece of work being undertaken by Network Rail on the North London Line as part of the £1.4 billion revamp of the London Overground, which is being jointly funded by TfL and the
Olympic Delivery Authority.
The bulk of the budget is being spent on extending the East London Line between Dalston in the north-east and West Croydon.
The extended East London line will become part of the Overground network when it opens in 2010 and will eventually link with the North London Line at Highbury and Islington in 2011, creating the beginning of an orbital service around the capital.To download a PDF of the planned northern line route, click on the resource box on the right hand side of the page