The contract, worth an initial £150m to Costain, starts immediately with design and professional services work, in collaboration with Network Rail, to develop detailed plans for the multi-disciplinary project.
The main works will take place between late 2014 and 2017.
As tipped in the Enquirer, Costain beat Balfour Beatty, Hochtief and VolkerFitzpatrick for the major station improvements and extensions package, which are being managed by Network Rail.
Major infrastructure improvements will also include upgrading the tracks and overhead electrification equipment as well as extra train stabling capacity and turnback facilities.
Work includes the design and build of major station improvements at Romford and Ilford.
Other station improvements include Forest Gate, Goodmayes, Harold Wood, Gidea Park, Chadwell Heath and Brentwood.
Crossrail will deliver significant benefits to passengers in east London and Essex including new trains, upgraded stations and quicker journeys.
Under the Crossrail programme, Network Rail is responsible for the upgrade of 43 miles of track in total, as well as improving 27 stations from Maidenhead in the west to Abbey Wood and Shenfield in the east.
Vinci Construction UK won the £100m job to design and upgrade 13 stations in west London and Berkshire, covering significant improvements to stations from Acton Main Line to Maidenhead.
Balfour Beatty took the £100m bundled job to upgrade lines and build Abeey Wood station on the shorter south east Spur of Crossrail.