A new dive-under in Acton which involved nearly 40,000 tonnes of concrete is near completion and will be in action early next year.
Network Rail began the construction in 2013, as part of the Crossrail programme, excavating 34,000 tonnes of earth from the five metre wide site.
Currently passenger services running through Acton are held up by long freight trains entering and leaving a freight yard, just west of Acton mainline station.
The new dive-under will allow Paddington -bound passenger services on the Great Western Main Line to pass under the slower freight trains to avoid delays which will increase capacity and improve reliability.
Matthew Steele, Crossrail Programme Director at Network Rail said: "This is another great achievement as part of Network Rail’s work on the Crossrail programme.
"Constructing the dive-under while trains continued to operate on either side of the work-site has been a massive engineering challenge.
"We will now work to ensure that the final elements of the dive-under are put into place so that trains can begin to use it from the start of 2017."
'Better and more reliable services'
More than 1,400 piled foundations have been installed and the construction has involved nearly 40,000 tonnes of concrete and the instillation of 730 metres of track.
Electric overhead wires to power trains and signals to control them will now be installed before the first test trains use the dive-under at the end of this year.
It is expected to be in full operation at the start of next year.
Matthew White, Surface Director at Crossrail, said: "The structural completion of the dive-under is another step towards providing people in west London and the Thames Valley with better and more reliable services as part of the Crossrail programme.
"The Elizabeth line will make it quicker and easier for people to get to a whole range of destinations across London and the South East."