Skanska has scooped a £105m deal to build a 1 km long box to accommodate the CTRL tracks at Stratford, east London, around which the international station will be built. It will also carry out preparation work for the new station.
Five firms are vying for a separate contract to build an extension to St Pancras Station, central London, for the Eurostar.
The shortlist comprises: Costain/Bachy Soletanche/O'Rourke; Balfour Beatty/Sir Robert McAlpine; Kier/Nuttall; Hochtief/Norwest Holst; and Skanska.
The client, Union Railways North, is due to award the contract in July, with a start on site next January. The contract involves civil and structural works for the St Pancras deck extension and the construction of a below-ground station for Thameslink services.