Sentence:  The 6.5km route is agreed from a junction with the relief lines of the Great Western main line to the west of Slough, the new link would <WSD>tunnel</WSD> under the M25 to reach Heathrow's Terminal 5 station, where space has been set aside to accommodate services from the west. 
Meaning of the word marked with <WSD> tags:  To make a tunnel through or under something; to burrow.