Today, 25 May 2021, Kiwirail has announced the opening of the deviation of the Main North Line at Tunnel 21 (Tar Barrel Tunnel) between Mirza and Wharanui. The part of the Main North Line between these locations was the historical Picton Section built in stages south from Picton between 1875 and 1915, and the railhead of the Section was located at at Wharanui, 6 km south of the tunnel, until 1942. The Picton Section originally incorporated three tunnels, No. 1, 2 and 3; the numbering was changed in 1974 to 23, 22 and 21. A fourth tunnel, No. 24 at Tuamarina, is an underpass of SH1 built about 60 years ago and was not part of the original construction of the Picton Section. Tunnels 21 to 23 have all been eliminated over a 32 year period, with the latter two daylighted in place in 1981 and 1979, respectively. When it came to Tunnel 21, Kiwirail chose to build a deviation in order to reduce the track curvature, and therefore left the tunnel itself in place, although the portals will be filled in with concrete.