A "dive-under" was constructed at Acton to allow passenger trains to pass slower freight trains leaving and entering a goods yard.
It was completed in July 2016 and was brought into use in 2017.
Crossrail is a 118-kilometre (73-mile) railway line under development in London and the home counties of Berkshire, Buckinghamshire and Essex, England.
The central section and a large portion of the line, between Paddington in central London and Abbey Wood in the south-east, is due to open in December 2018, whereupon that section and two other existing routes will be officially renamed the Elizabeth line after Queen Elizabeth II.
The central tunnels run from a portal just west of Paddington to Whitechapel, with further tunnelling to Stratford and to Canary Wharf.
There will be new stations at Paddington, Bond Street, Tottenham Court Road, Farringdon, Liverpool Street and Whitechapel, with interchanges with the London Underground and other National Rail services.
One of the two eastern sections runs underground from Whitechapel to Stratford, then on the surface on the existing main line.
Construction of a flyover near Hayes & Harlington station began in 2014, and will allow Heathrow Express trains to pass over the track used by Crossrail, avoiding delays caused by crossings.
It is expected to relieve pressure on existing east-west London Underground lines such as the Central and District lines, as well as the Jubilee line extension and the Heathrow branch of the Piccadilly line.
Crossrail will be operated by MTR Corporation (Crossrail) Ltd as a London Rail concession of Transport for London, The concept of large-diameter tunnels crossing central London to connect Paddington in the west and Liverpool Street in the east was first proposed by railwayman George Dow in The Star newspaper in June 1941.
At each end of the central core, the line will divide into two branches: in the east to Abbey Wood and Shenfield in Essex, and in the west to Heathrow Airport and Reading in Berkshire.
Part of one of the eastern branches, between Liverpool Street and Shenfield, was transferred to a precursor service called Tf L Rail in 2015; this precursor took control of the Heathrow Connect service in May 2018.