The 100-year-old bridge on Northolt Road was lifted out by a giant crane late last year.
The project team worked round-the-clock for four days to remove the old bridge and build the 460-tonne replacement.
Works included excavating approximately 2000 tonnes of existing abutments and embankment; placing six reinforced concrete crosshead breams and ballast walls; and installing two edge box girders and stiffened plate deck sections.
With more than an hour to spare, the team finished the bridge replacement, which is an integral part of the multi-million pound project to widen the highway and ease congestion at this local traffic pinch point.
The scheme is a collaboration between Harrow Council, Ealing Council, Transport for London and Network Rail, and partners EnterpriseMouchel and Birse Rail.