Contractors are carrying out work on the existing rail network that runs for three quarters of the route, above ground, through outer London, Berkshire and Essex.
Network Rail said passing the halfway point of improvement works marks a 'major milestone' for the £14.8bn Crossrail project.
Works include laying over 30 miles of new track, a series of station rebuilds, modernisation of signalling, and electrification of key parts of the railway to the west of London.
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Of the 29 stations being upgraded, nine are to be rebuilt with new ticket halls, while 16 will have their platforms extended.
Other additions include a new flyover in Hillingdon and a new dive-under tunnel beneath the railway at Acton.
Mark Carne, Network Rail chief executive, said: "As part of our Railway Upgrade Plan, we are delivering this vital component of the Crossrail programme on time and on budget.
"There is still a huge amount of work to do but we are on schedule to provide passengers in London and the South East with better connections, quicker, easier journeys and vastly improved stations."
Crossrail is due to open fully in 2019.
(LM)
