Petts Hill bridge, on the border of South Harrow and Ealing, has been replaced - a major step to overcoming congestion in the area.
The 100-year-old bridge in Northolt Road (A312) was lifted out on December 25 by a giant crane and its successor, which will leave more headroom for the traffic passing underneath, was constructed bit by bit the same day .
Engineers from Birse Rail worked round-the-clock for four days to remove the existing graffiti-covered bridge and build the 460-tonne replacement as part of a four year long, £8.8m traffic jam-busting project.
Network Rail had closed the Chiltern Lane, which terminates at Marylebone, between December 24 and December 27 and the constructors finished with more than an hour to spare.
Chiltern Railways' trains are now running again, although the road closures and trafic diversions will not end until Wednesday January 7.
The second stage, in which wider bus lanes, improved pavements and pedestrian crossing facilities, and extra traffic and cycle lanes, are introduced, will begin in spring 2009.
The scheme is a collaboration between Harrow Council, Ealing Council, Network Rail and Transport for London.
Councillor Susan Hall (Conservative), deputy leader of the council and portfolio holder for environment services, 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."
PETTS HILL BRIDGE SCHEME
- 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