First Crossrail tracks laid on Stockley Flyover bridge
The first section of track to be used by Crossrail trains has been laid on a new bridge in west London.
The 120m (131yd) long Stockley Flyover will be used by Heathrow Express and Heathrow Connect services from next year.
Crossrail services between Paddington and Heathrow are due to begin in 2018.
Crossrail said the new bridge was the largest single-span bridge installed on the Great Western railway since the days of Brunel.
It is hoped the bridge will ensure services to central London can join the Great Western Main Line without delays.
Matthew White, surface director at Crossrail, 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."
From late 2019, passengers will be able to travel from Heathrow through the centre of the London without changing trains, as Crossrail comes into full service.
Up to four Crossrail trains an hour are due to link Heathrow with central London.
Passengers are expected to be able to reach Bond Street in 26 minutes and Canary Wharf in 40 minutes.
Network Rail is responsible for the design, development and delivery of the parts of Crossrail that are on the existing network.
Matthew Steele, Crossrail programme director at Network Rail said: "Once Crossrail is operational, this brand new structure will enable us to increase the number of rail services between central London and Heathrow airport.
"After four years of planning is it great to see this new infrastructure being brought into service. Well done to everyone involved."
The Crossrail route will pass through 40 stations and run more than 100km (62 miles) from Reading and Heathrow in the west, through new twin-bore 21km (13-mile) tunnels below central London to Shenfield and Abbey Wood in the east.