Balfour Beatty will work on electrifying 12.5 miles of the Great Western Main Line between Stockley Junction and Maidenhead, starting in December and due to be finished in late 2016.
As well as preparing this portion of the track for Crossrail, the upgrade should also allow for faster intercity trains on the Great Western route, Crossrail said.
The £14.8bn cross-London train service will run from Maidenhead to Shenfield once it is fully open in 2019. Engineers have bored more than half of the 26 miles of tunnel needed for the route.
Balfour Beatty is already involved in building a new station at Abbey Wood and upgrading above-ground parts of the track.
Andrew McNaughton, the firm’s boss, said the contract continues “our long-term relationship with Network Rail and continued support with the Crossrail programme”.