The final beam has been lifted into place to support the construction of a second ramp at Stockley flyover as part of the Crossrail programme.
The work by Network Rail marked a major milestone in Hillingdon, with a total of 146 concrete beams, each weighing around 40 tonnes - more than three times the weight of a London bus - installed at the site.
Work has been carried out at night to avoid disruption to train services and was completed six weeks ahead of schedule.
The first section of the flyover in west London came into use for Heathrow Connect services at the end of 2014, and this second ramp is now being constructed so it can also be used by Heathrow Express trains.
Matthew White, Surface Director at Crossrail said: “Putting in place so many heavy beams so close to a running railway is no mean feat. Once the flyover is fully open it will help to provide great links between Heathrow and central London and contribute to London’s continued presence as a modern world-class city.”
The new flyover in Stockley, Hillingdon, will ensure Crossrail and Heathrow Express services heading towards central London will be able to join the Great Western Main Line without delaying, or being delayed, by other trains using the route.
When the route to the west of London fully opens in 2019, it will improve links between Heathrow and the capital's commercial and financial districts – the West End, the City and Canary Wharf.
Passengers will be able to board one train all the way through central London from Heathrow, reaching Bond Street in 26 minutes and Canary Wharf in 40 minutes.
Matthew Steele, Crossrail programme director at Network Rail said: “To complete this element of the work six weeks ahead of schedule, safely and with minimal impact on passengers is a great achievement.
"There is still a lot of work to be done, and we will now focus on completing the second section of the flyover, ready for Heathrow Express services.”