Carillion has won a £90 million contract from Network Rail to upgrade the North London Line.
The project will enhance the network between South Acton and Stratford/Barking to improve rail services, including connections with the new East London Railway, in preparation for the 2012 Olympic Games.
Carillion will be responsible for the design and construction of the civil engineering, track, overhead line and power supply aspects of the project.
The job forms part of a £1.4 billion upgrade of the London Overground railway, which is being jointly funded by Network Rail, Transport for London and the Olympic Delivery Authority.
Carillion chief executive John McDonough said: “This project is a key part of the programme to upgrade the London rail network and we are delighted to be working with Network Rail to deliver this project ahead of the 2012 Olympics.”