Carillion has laid the first section of track for the £14bn Crossrail scheme.
The contractor installed the rail on the Stockley Flyover in west London, which it is building as part of an upgrade project for Network Rail.
Heathrow Express and Heathrow Connect services will use the track for airport services from next year. Crossrail trains will also run on the line from 2018.
The 120m-long Stockley Flyover is the largest single-span bridge to have been installed anywhere on the Great Western railway since Brunel’s days.
Crossrail surface director Matthew White said: “Installing the first section of Crossrail track is a major milestone for the project.
“The flyover at Stockley will provide a vital new link between Heathrow and London, significantly improving connections between the airport and the West End, the City and Canary Wharf.”
The government earlier this month announced £14M funding to introduce step-free access at the three outstanding Crossrail stations.