The Western Rail Link from the Langley area to Heathrow Terminal 5 would be mostly tunnelled and is forecast to cost £700m-£900m.
Invitations to tender for three separate contract packages are due to be advertised in November 2019.
Its planned route leaves the main line between Langley and Iver and then descends underneath the main railway line into a cutting before entering a 5km tunnel.
The tunnel would pass under Richings Park and Colnbrook and then merge with existing rail lines underground at Heathrow Terminal 5.
The proposed construction programme will be based on three discrete packages of work where the design and construction of each package can be completed largely in isolation of the other packages.
The first package will consist of works to construct the Rail Intersection Bridge underneath the Great Western Main Line and will include all civil engineering, track, OLE and associated works to create both the bridge and the new rail junction with the busy rail route.
A second package will consist of the works to construct the two tunnels into T5 and will include the procurement and operation of the tunnel boring machines and the construction of the shafts, cross passages and sprayed concrete sections.
The third package will consist of the fit-out of the tunnel section, including track, OLE and ventilation.
A bidder workshop will be held in Swindon on the 23 July at 9:00 a.m. until 4:00 p.m. For further details email, WRLtHProcurement@networkrail.co.uk by Friday 5 July.