United Kingdom, Jan. 26 -- Construction works on HS2's longest tunnel have been completed - marking an important civil engineering milestone for the new high-speed railway.Close of work at two of the line's Chiltern tunnel vent shafts at Chesham Road and Little Missenden brings to an end a project that began almost five years ago.The Chiltern tunnel's construction was led by HS2 Ltd's main works contractor for this section of the line - Align JV, a joint venture between Sir Robert McAlpine, Bouygues Travaux Publics and VolkerFitzpatrick.Once fitted out with tracks and overhead electrical equipment as part of later phases of the project, the tunnel will carry trains travelling at 200mph, enabling them to traverse its 10 miles in just three m...