Trains will be cancelled for four days and traffic diverted for a fortnight while engingeers replace the Petts Hill Bridge this Christmas.
Swapping the railway bridge and widening the congestion-choked road beneath it, at a cost of £8.8million, has been in the pipeline for four years.
Campaigners living on the Harrow-Ealing border collected more than 5,000 signatures and lobbied relentlessly to ensure the bottleneck bridge was seen to.
Network Rail is closing the Chiltern line between December 24 and 27 - selected because it is the quietest and least disruptive time of the year - so that Harrow Council's construction firm Enterprisemouchel (CORR) can work solidly for the four days to lift out the 100-year-old bridge and drop its taller successor into place.
Meanwhile, road closures will force traffic, including 140, 487 and 398 bus routes, to be re-routed along signed diversions between Monday December 22 and Wednesday January 7 to allow the pre- and post-bridge swap work to be completed.
The second stage, whereby wider bus lanes, improved pavements and pedestrian crossing facilities, and extra traffic and cycle lanes, are introduced, will begin in spring 2009.
Harrow Council's portfolio holder for environment services, Councillor Susan Hall (Conservative), said: "Not only will this scheme bring relief to drivers tired of traffic jams, it will also make it safer for pedestrians and cyclists, and improve the railway and public transport links.
"We really need to thank everybody in the community for their support and patience, as we get closer to delivering this important project."
The scheme is a collaboration between Harrow Council, Ealing Council, Network Rail and Transport for London.
PETTS HILL BRIDGE SCHEME PLAN:
replace 100-year-old railway bridge with taller, wider one
extra traffic lane in each direction and a bus priority at the junction
improving access to Northolt Park station by widening footway and installing new cycle paths and lighting
widening pavements under bridge
creating cycle tracks to join up existing cycle lanes
upgrading the signal junction at A312 (Northolt Road) and A4090 (Alexandra Avenue) to provide crossing facilities
planting new trees