The new flyover will ensure that 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.
Two jacks launched the bridge with a force equivalent to that needed to lift 115 London taxis.
A total of 20 moveable wheels and special Teflon sheets were also used to help the bridge launch.
Matthew White, Surface Director at Crossrail said: “Launching a thousand tonne bridge over the Great Western Main Line is quite a feat of engineering and one that I’m sure would have made Brunel proud.
Rob McIntosh, Crossrail Programme Director, Network Rail said: “We used innovative methods as this was a complex and difficult operation.
“The system of sliding the bridge over the railway line allowed for controlled and safe installation with minimal interruption of rail services.
“That said, it took a lot of preparation ahead of time, and we monitored progress very carefully.”
Crossrail services between London Paddington and Heathrow will begin in 2018.
From 2019, the full service will begin, allowing passengers to travel from Heathrow right through the centre of the capital without changing trains.