The firm beat rivals Vinci and Laing O’Rourke to the job, which was originally estimated to cost £70m before being rescoped.
As part of the big Reading station upgrade, which Costain has secured, a new east railway chord will be built through the existing train care depot, to allow for grade separation of the main lines.
Work building a new train care depot on railway land to the north west of the station should begin in January.